FreeBSD/Linux Kernel Cross Reference
sys/kernel/futex.c

     /*
      *  Fast Userspace Mutexes (which I call "Futexes!").
      *  (C) Rusty Russell, IBM 2002
      *
      *  Generalized futexes, futex requeueing, misc fixes by Ingo Molnar
      *  (C) Copyright 2003 Red Hat Inc, All Rights Reserved
      *
      *  Removed page pinning, fix privately mapped COW pages and other cleanups
      *  (C) Copyright 2003, 2004 Jamie Lokier
     *
     *  Robust futex support started by Ingo Molnar
     *  (C) Copyright 2006 Red Hat Inc, All Rights Reserved
     *  Thanks to Thomas Gleixner for suggestions, analysis and fixes.
     *
     *  PI-futex support started by Ingo Molnar and Thomas Gleixner
     *  Copyright (C) 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
     *  Copyright (C) 2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com>
     *
     *  PRIVATE futexes by Eric Dumazet
     *  Copyright (C) 2007 Eric Dumazet <dada1@cosmosbay.com>
     *
     *  Requeue-PI support by Darren Hart <dvhltc@us.ibm.com>
     *  Copyright (C) IBM Corporation, 2009
     *  Thanks to Thomas Gleixner for conceptual design and careful reviews.
     *
     *  Thanks to Ben LaHaise for yelling "hashed waitqueues" loudly
     *  enough at me, Linus for the original (flawed) idea, Matthew
     *  Kirkwood for proof-of-concept implementation.
     *
     *  "The futexes are also cursed."
     *  "But they come in a choice of three flavours!"
     *
     *  This program is free software; you can redistribute it and/or modify
     *  it under the terms of the GNU General Public License as published by
     *  the Free Software Foundation; either version 2 of the License, or
     *  (at your option) any later version.
     *
     *  This program is distributed in the hope that it will be useful,
     *  but WITHOUT ANY WARRANTY; without even the implied warranty of
     *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     *  GNU General Public License for more details.
     *
     *  You should have received a copy of the GNU General Public License
     *  along with this program; if not, write to the Free Software
     *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
     */
    #include <linux/slab.h>
    #include <linux/poll.h>
    #include <linux/fs.h>
    #include <linux/file.h>
    #include <linux/jhash.h>
    #include <linux/init.h>
    #include <linux/futex.h>
    #include <linux/mount.h>
    #include <linux/pagemap.h>
    #include <linux/syscalls.h>
    #include <linux/signal.h>
    #include <linux/export.h>
    #include <linux/magic.h>
    #include <linux/pid.h>
    #include <linux/nsproxy.h>
    #include <linux/ptrace.h>
    
    #include <asm/futex.h>
    
    #include "rtmutex_common.h"
    
    int __read_mostly futex_cmpxchg_enabled;
    
    #define FUTEX_HASHBITS (CONFIG_BASE_SMALL ? 4 : 8)
    
    /*
     * Futex flags used to encode options to functions and preserve them across
     * restarts.
     */
    #define FLAGS_SHARED            0x01
    #define FLAGS_CLOCKRT           0x02
    #define FLAGS_HAS_TIMEOUT       0x04
    
    /*
     * Priority Inheritance state:
     */
    struct futex_pi_state {
            /*
             * list of 'owned' pi_state instances - these have to be
             * cleaned up in do_exit() if the task exits prematurely:
             */
            struct list_head list;
    
            /*
             * The PI object:
             */
            struct rt_mutex pi_mutex;
    
            struct task_struct *owner;
            atomic_t refcount;
    
            union futex_key key;
    };
   
   /**
    * struct futex_q - The hashed futex queue entry, one per waiting task
    * @list:               priority-sorted list of tasks waiting on this futex
    * @task:               the task waiting on the futex
    * @lock_ptr:           the hash bucket lock
    * @key:                the key the futex is hashed on
    * @pi_state:           optional priority inheritance state
    * @rt_waiter:          rt_waiter storage for use with requeue_pi
    * @requeue_pi_key:     the requeue_pi target futex key
    * @bitset:             bitset for the optional bitmasked wakeup
    *
    * We use this hashed waitqueue, instead of a normal wait_queue_t, so
    * we can wake only the relevant ones (hashed queues may be shared).
    *
    * A futex_q has a woken state, just like tasks have TASK_RUNNING.
    * It is considered woken when plist_node_empty(&q->list) || q->lock_ptr == 0.
    * The order of wakeup is always to make the first condition true, then
    * the second.
    *
    * PI futexes are typically woken before they are removed from the hash list via
    * the rt_mutex code. See unqueue_me_pi().
    */
   struct futex_q {
           struct plist_node list;
   
           struct task_struct *task;
           spinlock_t *lock_ptr;
           union futex_key key;
           struct futex_pi_state *pi_state;
           struct rt_mutex_waiter *rt_waiter;
           union futex_key *requeue_pi_key;
           u32 bitset;
   };
   
   static const struct futex_q futex_q_init = {
           /* list gets initialized in queue_me()*/
           .key = FUTEX_KEY_INIT,
           .bitset = FUTEX_BITSET_MATCH_ANY
   };
   
   /*
    * Hash buckets are shared by all the futex_keys that hash to the same
    * location.  Each key may have multiple futex_q structures, one for each task
    * waiting on a futex.
    */
   struct futex_hash_bucket {
           spinlock_t lock;
           struct plist_head chain;
   };
   
   static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS];
   
   /*
    * We hash on the keys returned from get_futex_key (see below).
    */
   static struct futex_hash_bucket *hash_futex(union futex_key *key)
   {
           u32 hash = jhash2((u32*)&key->both.word,
                             (sizeof(key->both.word)+sizeof(key->both.ptr))/4,
                             key->both.offset);
           return &futex_queues[hash & ((1 << FUTEX_HASHBITS)-1)];
   }
   
   /*
    * Return 1 if two futex_keys are equal, 0 otherwise.
    */
   static inline int match_futex(union futex_key *key1, union futex_key *key2)
   {
           return (key1 && key2
                   && key1->both.word == key2->both.word
                   && key1->both.ptr == key2->both.ptr
                   && key1->both.offset == key2->both.offset);
   }
   
   /*
    * Take a reference to the resource addressed by a key.
    * Can be called while holding spinlocks.
    *
    */
   static void get_futex_key_refs(union futex_key *key)
   {
           if (!key->both.ptr)
                   return;
   
           switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
           case FUT_OFF_INODE:
                   ihold(key->shared.inode);
                   break;
           case FUT_OFF_MMSHARED:
                   atomic_inc(&key->private.mm->mm_count);
                   break;
           }
   }
   
   /*
    * Drop a reference to the resource addressed by a key.
    * The hash bucket spinlock must not be held.
    */
   static void drop_futex_key_refs(union futex_key *key)
   {
           if (!key->both.ptr) {
                   /* If we're here then we tried to put a key we failed to get */
                   WARN_ON_ONCE(1);
                   return;
           }
   
           switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
           case FUT_OFF_INODE:
                   iput(key->shared.inode);
                   break;
           case FUT_OFF_MMSHARED:
                   mmdrop(key->private.mm);
                   break;
           }
   }
   
   /**
    * get_futex_key() - Get parameters which are the keys for a futex
    * @uaddr:      virtual address of the futex
    * @fshared:    0 for a PROCESS_PRIVATE futex, 1 for PROCESS_SHARED
    * @key:        address where result is stored.
    * @rw:         mapping needs to be read/write (values: VERIFY_READ,
    *              VERIFY_WRITE)
    *
    * Returns a negative error code or 0
    * The key words are stored in *key on success.
    *
    * For shared mappings, it's (page->index, vma->vm_file->f_path.dentry->d_inode,
    * offset_within_page).  For private mappings, it's (uaddr, current->mm).
    * We can usually work out the index without swapping in the page.
    *
    * lock_page() might sleep, the caller should not hold a spinlock.
    */
   static int
   get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
   {
           unsigned long address = (unsigned long)uaddr;
           struct mm_struct *mm = current->mm;
           struct page *page, *page_head;
           int err, ro = 0;
   
           /*
            * The futex address must be "naturally" aligned.
            */
           key->both.offset = address % PAGE_SIZE;
           if (unlikely((address % sizeof(u32)) != 0))
                   return -EINVAL;
           address -= key->both.offset;
   
           /*
            * PROCESS_PRIVATE futexes are fast.
            * As the mm cannot disappear under us and the 'key' only needs
            * virtual address, we dont even have to find the underlying vma.
            * Note : We do have to check 'uaddr' is a valid user address,
            *        but access_ok() should be faster than find_vma()
            */
           if (!fshared) {
                   if (unlikely(!access_ok(VERIFY_WRITE, uaddr, sizeof(u32))))
                           return -EFAULT;
                   key->private.mm = mm;
                   key->private.address = address;
                   get_futex_key_refs(key);
                   return 0;
           }
   
   again:
           err = get_user_pages_fast(address, 1, 1, &page);
           /*
            * If write access is not required (eg. FUTEX_WAIT), try
            * and get read-only access.
            */
           if (err == -EFAULT && rw == VERIFY_READ) {
                   err = get_user_pages_fast(address, 1, 0, &page);
                   ro = 1;
           }
           if (err < 0)
                   return err;
           else
                   err = 0;
   
   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
           page_head = page;
           if (unlikely(PageTail(page))) {
                   put_page(page);
                   /* serialize against __split_huge_page_splitting() */
                   local_irq_disable();
                   if (likely(__get_user_pages_fast(address, 1, 1, &page) == 1)) {
                           page_head = compound_head(page);
                           /*
                            * page_head is valid pointer but we must pin
                            * it before taking the PG_lock and/or
                            * PG_compound_lock. The moment we re-enable
                            * irqs __split_huge_page_splitting() can
                            * return and the head page can be freed from
                            * under us. We can't take the PG_lock and/or
                            * PG_compound_lock on a page that could be
                            * freed from under us.
                            */
                           if (page != page_head) {
                                   get_page(page_head);
                                   put_page(page);
                           }
                           local_irq_enable();
                   } else {
                           local_irq_enable();
                           goto again;
                   }
           }
   #else
           page_head = compound_head(page);
           if (page != page_head) {
                   get_page(page_head);
                   put_page(page);
           }
   #endif
   
           lock_page(page_head);
   
           /*
            * If page_head->mapping is NULL, then it cannot be a PageAnon
            * page; but it might be the ZERO_PAGE or in the gate area or
            * in a special mapping (all cases which we are happy to fail);
            * or it may have been a good file page when get_user_pages_fast
            * found it, but truncated or holepunched or subjected to
            * invalidate_complete_page2 before we got the page lock (also
            * cases which we are happy to fail).  And we hold a reference,
            * so refcount care in invalidate_complete_page's remove_mapping
            * prevents drop_caches from setting mapping to NULL beneath us.
            *
            * The case we do have to guard against is when memory pressure made
            * shmem_writepage move it from filecache to swapcache beneath us:
            * an unlikely race, but we do need to retry for page_head->mapping.
            */
           if (!page_head->mapping) {
                   int shmem_swizzled = PageSwapCache(page_head);
                   unlock_page(page_head);
                   put_page(page_head);
                   if (shmem_swizzled)
                           goto again;
                   return -EFAULT;
           }
   
           /*
            * Private mappings are handled in a simple way.
            *
            * NOTE: When userspace waits on a MAP_SHARED mapping, even if
            * it's a read-only handle, it's expected that futexes attach to
            * the object not the particular process.
            */
           if (PageAnon(page_head)) {
                   /*
                    * A RO anonymous page will never change and thus doesn't make
                    * sense for futex operations.
                    */
                   if (ro) {
                           err = -EFAULT;
                           goto out;
                   }
   
                   key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
                   key->private.mm = mm;
                   key->private.address = address;
           } else {
                   key->both.offset |= FUT_OFF_INODE; /* inode-based key */
                   key->shared.inode = page_head->mapping->host;
                   key->shared.pgoff = page_head->index;
           }
   
           get_futex_key_refs(key);
   
   out:
           unlock_page(page_head);
           put_page(page_head);
           return err;
   }
   
   static inline void put_futex_key(union futex_key *key)
   {
           drop_futex_key_refs(key);
   }
   
   /**
    * fault_in_user_writeable() - Fault in user address and verify RW access
    * @uaddr:      pointer to faulting user space address
    *
    * Slow path to fixup the fault we just took in the atomic write
    * access to @uaddr.
    *
    * We have no generic implementation of a non-destructive write to the
    * user address. We know that we faulted in the atomic pagefault
    * disabled section so we can as well avoid the #PF overhead by
    * calling get_user_pages() right away.
    */
   static int fault_in_user_writeable(u32 __user *uaddr)
   {
           struct mm_struct *mm = current->mm;
           int ret;
   
           down_read(&mm->mmap_sem);
           ret = fixup_user_fault(current, mm, (unsigned long)uaddr,
                                  FAULT_FLAG_WRITE);
           up_read(&mm->mmap_sem);
   
           return ret < 0 ? ret : 0;
   }
   
   /**
    * futex_top_waiter() - Return the highest priority waiter on a futex
    * @hb:         the hash bucket the futex_q's reside in
    * @key:        the futex key (to distinguish it from other futex futex_q's)
    *
    * Must be called with the hb lock held.
    */
   static struct futex_q *futex_top_waiter(struct futex_hash_bucket *hb,
                                           union futex_key *key)
   {
           struct futex_q *this;
   
           plist_for_each_entry(this, &hb->chain, list) {
                   if (match_futex(&this->key, key))
                           return this;
           }
           return NULL;
   }
   
   static int cmpxchg_futex_value_locked(u32 *curval, u32 __user *uaddr,
                                         u32 uval, u32 newval)
   {
           int ret;
   
           pagefault_disable();
           ret = futex_atomic_cmpxchg_inatomic(curval, uaddr, uval, newval);
           pagefault_enable();
   
           return ret;
   }
   
   static int get_futex_value_locked(u32 *dest, u32 __user *from)
   {
           int ret;
   
           pagefault_disable();
           ret = __copy_from_user_inatomic(dest, from, sizeof(u32));
           pagefault_enable();
   
           return ret ? -EFAULT : 0;
   }
   
   
   /*
    * PI code:
    */
   static int refill_pi_state_cache(void)
   {
           struct futex_pi_state *pi_state;
   
           if (likely(current->pi_state_cache))
                   return 0;
   
           pi_state = kzalloc(sizeof(*pi_state), GFP_KERNEL);
   
           if (!pi_state)
                   return -ENOMEM;
   
           INIT_LIST_HEAD(&pi_state->list);
           /* pi_mutex gets initialized later */
           pi_state->owner = NULL;
           atomic_set(&pi_state->refcount, 1);
           pi_state->key = FUTEX_KEY_INIT;
   
           current->pi_state_cache = pi_state;
   
           return 0;
   }
   
   static struct futex_pi_state * alloc_pi_state(void)
   {
           struct futex_pi_state *pi_state = current->pi_state_cache;
   
           WARN_ON(!pi_state);
           current->pi_state_cache = NULL;
   
           return pi_state;
   }
   
   static void free_pi_state(struct futex_pi_state *pi_state)
   {
           if (!atomic_dec_and_test(&pi_state->refcount))
                   return;
   
           /*
            * If pi_state->owner is NULL, the owner is most probably dying
            * and has cleaned up the pi_state already
            */
           if (pi_state->owner) {
                   raw_spin_lock_irq(&pi_state->owner->pi_lock);
                   list_del_init(&pi_state->list);
                   raw_spin_unlock_irq(&pi_state->owner->pi_lock);
   
                   rt_mutex_proxy_unlock(&pi_state->pi_mutex, pi_state->owner);
           }
   
           if (current->pi_state_cache)
                   kfree(pi_state);
           else {
                   /*
                    * pi_state->list is already empty.
                    * clear pi_state->owner.
                    * refcount is at 0 - put it back to 1.
                    */
                   pi_state->owner = NULL;
                   atomic_set(&pi_state->refcount, 1);
                   current->pi_state_cache = pi_state;
           }
   }
   
   /*
    * Look up the task based on what TID userspace gave us.
    * We dont trust it.
    */
   static struct task_struct * futex_find_get_task(pid_t pid)
   {
           struct task_struct *p;
   
           rcu_read_lock();
           p = find_task_by_vpid(pid);
           if (p)
                   get_task_struct(p);
   
           rcu_read_unlock();
   
           return p;
   }
   
   /*
    * This task is holding PI mutexes at exit time => bad.
    * Kernel cleans up PI-state, but userspace is likely hosed.
    * (Robust-futex cleanup is separate and might save the day for userspace.)
    */
   void exit_pi_state_list(struct task_struct *curr)
   {
           struct list_head *next, *head = &curr->pi_state_list;
           struct futex_pi_state *pi_state;
           struct futex_hash_bucket *hb;
           union futex_key key = FUTEX_KEY_INIT;
   
           if (!futex_cmpxchg_enabled)
                   return;
           /*
            * We are a ZOMBIE and nobody can enqueue itself on
            * pi_state_list anymore, but we have to be careful
            * versus waiters unqueueing themselves:
            */
           raw_spin_lock_irq(&curr->pi_lock);
           while (!list_empty(head)) {
   
                   next = head->next;
                   pi_state = list_entry(next, struct futex_pi_state, list);
                   key = pi_state->key;
                   hb = hash_futex(&key);
                   raw_spin_unlock_irq(&curr->pi_lock);
   
                   spin_lock(&hb->lock);
   
                   raw_spin_lock_irq(&curr->pi_lock);
                   /*
                    * We dropped the pi-lock, so re-check whether this
                    * task still owns the PI-state:
                    */
                   if (head->next != next) {
                           spin_unlock(&hb->lock);
                           continue;
                   }
   
                   WARN_ON(pi_state->owner != curr);
                   WARN_ON(list_empty(&pi_state->list));
                   list_del_init(&pi_state->list);
                   pi_state->owner = NULL;
                   raw_spin_unlock_irq(&curr->pi_lock);
   
                   rt_mutex_unlock(&pi_state->pi_mutex);
   
                   spin_unlock(&hb->lock);
   
                   raw_spin_lock_irq(&curr->pi_lock);
           }
           raw_spin_unlock_irq(&curr->pi_lock);
   }
   
   static int
   lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
                   union futex_key *key, struct futex_pi_state **ps)
   {
           struct futex_pi_state *pi_state = NULL;
           struct futex_q *this, *next;
           struct plist_head *head;
           struct task_struct *p;
           pid_t pid = uval & FUTEX_TID_MASK;
   
           head = &hb->chain;
   
           plist_for_each_entry_safe(this, next, head, list) {
                   if (match_futex(&this->key, key)) {
                           /*
                            * Another waiter already exists - bump up
                            * the refcount and return its pi_state:
                            */
                           pi_state = this->pi_state;
                           /*
                            * Userspace might have messed up non-PI and PI futexes
                            */
                           if (unlikely(!pi_state))
                                   return -EINVAL;
   
                           WARN_ON(!atomic_read(&pi_state->refcount));
   
                           /*
                            * When pi_state->owner is NULL then the owner died
                            * and another waiter is on the fly. pi_state->owner
                            * is fixed up by the task which acquires
                            * pi_state->rt_mutex.
                            *
                            * We do not check for pid == 0 which can happen when
                            * the owner died and robust_list_exit() cleared the
                            * TID.
                            */
                           if (pid && pi_state->owner) {
                                   /*
                                    * Bail out if user space manipulated the
                                    * futex value.
                                    */
                                   if (pid != task_pid_vnr(pi_state->owner))
                                           return -EINVAL;
                           }
   
                           atomic_inc(&pi_state->refcount);
                           *ps = pi_state;
   
                           return 0;
                   }
           }
   
           /*
            * We are the first waiter - try to look up the real owner and attach
            * the new pi_state to it, but bail out when TID = 0
            */
           if (!pid)
                   return -ESRCH;
           p = futex_find_get_task(pid);
           if (!p)
                   return -ESRCH;
   
           /*
            * We need to look at the task state flags to figure out,
            * whether the task is exiting. To protect against the do_exit
            * change of the task flags, we do this protected by
            * p->pi_lock:
            */
           raw_spin_lock_irq(&p->pi_lock);
           if (unlikely(p->flags & PF_EXITING)) {
                   /*
                    * The task is on the way out. When PF_EXITPIDONE is
                    * set, we know that the task has finished the
                    * cleanup:
                    */
                   int ret = (p->flags & PF_EXITPIDONE) ? -ESRCH : -EAGAIN;
   
                   raw_spin_unlock_irq(&p->pi_lock);
                   put_task_struct(p);
                   return ret;
           }
   
           pi_state = alloc_pi_state();
   
           /*
            * Initialize the pi_mutex in locked state and make 'p'
            * the owner of it:
            */
           rt_mutex_init_proxy_locked(&pi_state->pi_mutex, p);
   
           /* Store the key for possible exit cleanups: */
           pi_state->key = *key;
   
           WARN_ON(!list_empty(&pi_state->list));
           list_add(&pi_state->list, &p->pi_state_list);
           pi_state->owner = p;
           raw_spin_unlock_irq(&p->pi_lock);
   
           put_task_struct(p);
   
           *ps = pi_state;
   
           return 0;
   }
   
   /**
    * futex_lock_pi_atomic() - Atomic work required to acquire a pi aware futex
    * @uaddr:              the pi futex user address
    * @hb:                 the pi futex hash bucket
    * @key:                the futex key associated with uaddr and hb
    * @ps:                 the pi_state pointer where we store the result of the
    *                      lookup
    * @task:               the task to perform the atomic lock work for.  This will
    *                      be "current" except in the case of requeue pi.
    * @set_waiters:        force setting the FUTEX_WAITERS bit (1) or not (0)
    *
    * Returns:
    *  0 - ready to wait
    *  1 - acquired the lock
    * <0 - error
    *
    * The hb->lock and futex_key refs shall be held by the caller.
    */
   static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
                                   union futex_key *key,
                                   struct futex_pi_state **ps,
                                   struct task_struct *task, int set_waiters)
   {
           int lock_taken, ret, force_take = 0;
           u32 uval, newval, curval, vpid = task_pid_vnr(task);
   
   retry:
           ret = lock_taken = 0;
   
           /*
            * To avoid races, we attempt to take the lock here again
            * (by doing a 0 -> TID atomic cmpxchg), while holding all
            * the locks. It will most likely not succeed.
            */
           newval = vpid;
           if (set_waiters)
                   newval |= FUTEX_WAITERS;
   
           if (unlikely(cmpxchg_futex_value_locked(&curval, uaddr, 0, newval)))
                   return -EFAULT;
   
           /*
            * Detect deadlocks.
            */
           if ((unlikely((curval & FUTEX_TID_MASK) == vpid)))
                   return -EDEADLK;
   
           /*
            * Surprise - we got the lock. Just return to userspace:
            */
           if (unlikely(!curval))
                   return 1;
   
           uval = curval;
   
           /*
            * Set the FUTEX_WAITERS flag, so the owner will know it has someone
            * to wake at the next unlock.
            */
           newval = curval | FUTEX_WAITERS;
   
           /*
            * Should we force take the futex? See below.
            */
           if (unlikely(force_take)) {
                   /*
                    * Keep the OWNER_DIED and the WAITERS bit and set the
                    * new TID value.
                    */
                   newval = (curval & ~FUTEX_TID_MASK) | vpid;
                   force_take = 0;
                   lock_taken = 1;
           }
   
           if (unlikely(cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)))
                   return -EFAULT;
           if (unlikely(curval != uval))
                   goto retry;
   
           /*
            * We took the lock due to forced take over.
            */
           if (unlikely(lock_taken))
                   return 1;
   
           /*
            * We dont have the lock. Look up the PI state (or create it if
            * we are the first waiter):
            */
           ret = lookup_pi_state(uval, hb, key, ps);
   
           if (unlikely(ret)) {
                   switch (ret) {
                   case -ESRCH:
                           /*
                            * We failed to find an owner for this
                            * futex. So we have no pi_state to block
                            * on. This can happen in two cases:
                            *
                            * 1) The owner died
                            * 2) A stale FUTEX_WAITERS bit
                            *
                            * Re-read the futex value.
                            */
                           if (get_futex_value_locked(&curval, uaddr))
                                   return -EFAULT;
   
                           /*
                            * If the owner died or we have a stale
                            * WAITERS bit the owner TID in the user space
                            * futex is 0.
                            */
                           if (!(curval & FUTEX_TID_MASK)) {
                                   force_take = 1;
                                   goto retry;
                           }
                   default:
                           break;
                   }
           }
   
           return ret;
   }
   
   /**
    * __unqueue_futex() - Remove the futex_q from its futex_hash_bucket
    * @q:  The futex_q to unqueue
    *
    * The q->lock_ptr must not be NULL and must be held by the caller.
    */
   static void __unqueue_futex(struct futex_q *q)
   {
           struct futex_hash_bucket *hb;
   
           if (WARN_ON_SMP(!q->lock_ptr || !spin_is_locked(q->lock_ptr))
               || WARN_ON(plist_node_empty(&q->list)))
                   return;
   
           hb = container_of(q->lock_ptr, struct futex_hash_bucket, lock);
           plist_del(&q->list, &hb->chain);
   }
   
   /*
    * The hash bucket lock must be held when this is called.
    * Afterwards, the futex_q must not be accessed.
    */
   static void wake_futex(struct futex_q *q)
   {
           struct task_struct *p = q->task;
   
           if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
                   return;
   
           /*
            * We set q->lock_ptr = NULL _before_ we wake up the task. If
            * a non-futex wake up happens on another CPU then the task
            * might exit and p would dereference a non-existing task
            * struct. Prevent this by holding a reference on p across the
            * wake up.
            */
           get_task_struct(p);
   
           __unqueue_futex(q);
           /*
            * The waiting task can free the futex_q as soon as
            * q->lock_ptr = NULL is written, without taking any locks. A
            * memory barrier is required here to prevent the following
            * store to lock_ptr from getting ahead of the plist_del.
            */
           smp_wmb();
           q->lock_ptr = NULL;
   
           wake_up_state(p, TASK_NORMAL);
           put_task_struct(p);
   }
   
   static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this)
   {
           struct task_struct *new_owner;
           struct futex_pi_state *pi_state = this->pi_state;
           u32 uninitialized_var(curval), newval;
   
           if (!pi_state)
                   return -EINVAL;
   
           /*
            * If current does not own the pi_state then the futex is
            * inconsistent and user space fiddled with the futex value.
            */
           if (pi_state->owner != current)
                   return -EINVAL;
   
           raw_spin_lock(&pi_state->pi_mutex.wait_lock);
           new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
   
           /*
            * It is possible that the next waiter (the one that brought
            * this owner to the kernel) timed out and is no longer
            * waiting on the lock.
            */
           if (!new_owner)
                   new_owner = this->task;
   
           /*
            * We pass it to the next owner. (The WAITERS bit is always
            * kept enabled while there is PI state around. We must also
            * preserve the owner died bit.)
            */
           if (!(uval & FUTEX_OWNER_DIED)) {
                   int ret = 0;
   
                   newval = FUTEX_WAITERS | task_pid_vnr(new_owner);
   
                   if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval))
                           ret = -EFAULT;
                   else if (curval != uval)
                           ret = -EINVAL;
                   if (ret) {
                           raw_spin_unlock(&pi_state->pi_mutex.wait_lock);
                           return ret;
                   }
           }
   
           raw_spin_lock_irq(&pi_state->owner->pi_lock);
           WARN_ON(list_empty(&pi_state->list));
           list_del_init(&pi_state->list);
           raw_spin_unlock_irq(&pi_state->owner->pi_lock);
   
           raw_spin_lock_irq(&new_owner->pi_lock);
           WARN_ON(!list_empty(&pi_state->list));
           list_add(&pi_state->list, &new_owner->pi_state_list);
           pi_state->owner = new_owner;
           raw_spin_unlock_irq(&new_owner->pi_lock);
   
           raw_spin_unlock(&pi_state->pi_mutex.wait_lock);
           rt_mutex_unlock(&pi_state->pi_mutex);
   
           return 0;
   }
   
   static int unlock_futex_pi(u32 __user *uaddr, u32 uval)
   {
           u32 uninitialized_var(oldval);
   
           /*
            * There is no waiter, so we unlock the futex. The owner died
            * bit has not to be preserved here. We are the owner:
            */
           if (cmpxchg_futex_value_locked(&oldval, uaddr, uval, 0))
                   return -EFAULT;
           if (oldval != uval)
                   return -EAGAIN;
   
           return 0;
   }
   
   /*
    * Express the locking dependencies for lockdep:
    */
   static inline void
   double_lock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
   {
           if (hb1 <= hb2) {
                   spin_lock(&hb1->lock);
                   if (hb1 < hb2)
                           spin_lock_nested(&hb2->lock, SINGLE_DEPTH_NESTING);
           } else { /* hb1 > hb2 */
                   spin_lock(&hb2->lock);
                   spin_lock_nested(&hb1->lock, SINGLE_DEPTH_NESTING);
           }
   }
   
   static inline void
   double_unlock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
   {
           spin_unlock(&hb1->lock);
           if (hb1 != hb2)
                   spin_unlock(&hb2->lock);
   }
   
   /*
    * Wake up waiters matching bitset queued on this futex (uaddr).
    */
   static int
   futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
   {
           struct futex_hash_bucket *hb;
           struct futex_q *this, *next;
           struct plist_head *head;
           union futex_key key = FUTEX_KEY_INIT;
           int ret;
   
           if (!bitset)
                   return -EINVAL;
   
           ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, VERIFY_READ);
           if (unlikely(ret != 0))
                   goto out;
   
           hb = hash_futex(&key);
           spin_lock(&hb->lock);
           head = &hb->chain;
   
           plist_for_each_entry_safe(this, next, head, list) {
                  if (match_futex (&this->key, &key)) {
                          if (this->pi_state || this->rt_waiter) {
                                  ret = -EINVAL;
                                  break;
                          }
  
                          /* Check if one of the bits is set in both bitsets */
                          if (!(this->bitset & bitset))
                                  continue;
  
                          wake_futex(this);
                          if (++ret >= nr_wake)
                                  break;
                  }
          }
  
          spin_unlock(&hb->lock);
          put_futex_key(&key);
  out:
          return ret;
  }
  
  /*
   * Wake up all waiters hashed on the physical page that is mapped
   * to this virtual address:
   */
  static int
  futex_wake_op(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
                int nr_wake, int nr_wake2, int op)
  {
          union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
          struct futex_hash_bucket *hb1, *hb2;
          struct plist_head *head;
          struct futex_q *this, *next;
          int ret, op_ret;
  
  retry:
          ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, VERIFY_READ);
          if (unlikely(ret != 0))
                  goto out;
          ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, VERIFY_WRITE);
          if (unlikely(ret != 0))
                  goto out_put_key1;
  
          hb1 = hash_futex(&key1);
          hb2 = hash_futex(&key2);
  
  retry_private:
          double_lock_hb(hb1, hb2);
          op_ret = futex_atomic_op_inuser(op, uaddr2);
          if (unlikely(op_ret < 0)) {
  
                  double_unlock_hb(hb1, hb2);
  
  #ifndef CONFIG_MMU
                  /*
                   * we don't get EFAULT from MMU faults if we don't have an MMU,
                   * but we might get them from range checking
                   */
                  ret = op_ret;
                  goto out_put_keys;
  #endif
  
                  if (unlikely(op_ret != -EFAULT)) {
                          ret = op_ret;
                          goto out_put_keys;
                  }
  
                  ret = fault_in_user_writeable(uaddr2);
                  if (ret)
                          goto out_put_keys;
  
                  if (!(flags & FLAGS_SHARED))
                          goto retry_private;
  
                  put_futex_key(&key2);
                  put_futex_key(&key1);
                  goto retry;
          }
  
          head = &hb1->chain;
  
          plist_for_each_entry_safe(this, next, head, list) {
                  if (match_futex (&this->key, &key1)) {
                          if (this->pi_state || this->rt_waiter) {
                                  ret = -EINVAL;
                                  goto out_unlock;
                          }
                          wake_futex(this);
                          if (++ret >= nr_wake)
                                  break;
                  }
          }
  
          if (op_ret > 0) {
                  head = &hb2->chain;
  
                  op_ret = 0;
                  plist_for_each_entry_safe(this, next, head, list) {
                          if (match_futex (&this->key, &key2)) {
                                  if (this->pi_state || this->rt_waiter) {
                                          ret = -EINVAL;
                                          goto out_unlock;
                                  }
                                  wake_futex(this);
                                  if (++op_ret >= nr_wake2)
                                          break;
                          }
                  }
                  ret += op_ret;
          }
  
  out_unlock:
          double_unlock_hb(hb1, hb2);
  out_put_keys:
          put_futex_key(&key2);
  out_put_key1:
          put_futex_key(&key1);
  out:
          return ret;
  }
  
  /**
   * requeue_futex() - Requeue a futex_q from one hb to another
   * @q:          the futex_q to requeue
   * @hb1:        the source hash_bucket
   * @hb2:        the target hash_bucket
   * @key2:       the new key for the requeued futex_q
   */
  static inline
  void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1,
                     struct futex_hash_bucket *hb2, union futex_key *key2)
  {
  
          /*
           * If key1 and key2 hash to the same bucket, no need to
           * requeue.
           */
          if (likely(&hb1->chain != &hb2->chain)) {
                  plist_del(&q->list, &hb1->chain);
                  plist_add(&q->list, &hb2->chain);
                  q->lock_ptr = &hb2->lock;
          }
          get_futex_key_refs(key2);
          q->key = *key2;
  }
  
  /**
   * requeue_pi_wake_futex() - Wake a task that acquired the lock during requeue
   * @q:          the futex_q
   * @key:        the key of the requeue target futex
   * @hb:         the hash_bucket of the requeue target futex
   *
   * During futex_requeue, with requeue_pi=1, it is possible to acquire the
   * target futex if it is uncontended or via a lock steal.  Set the futex_q key
   * to the requeue target futex so the waiter can detect the wakeup on the right
   * futex, but remove it from the hb and NULL the rt_waiter so it can detect
   * atomic lock acquisition.  Set the q->lock_ptr to the requeue target hb->lock
   * to protect access to the pi_state to fixup the owner later.  Must be called
   * with both q->lock_ptr and hb->lock held.
   */
  static inline
  void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
                             struct futex_hash_bucket *hb)
  {
          get_futex_key_refs(key);
          q->key = *key;
  
          __unqueue_futex(q);
  
          WARN_ON(!q->rt_waiter);
          q->rt_waiter = NULL;
  
          q->lock_ptr = &hb->lock;
  
          wake_up_state(q->task, TASK_NORMAL);
  }
  
  /**
   * futex_proxy_trylock_atomic() - Attempt an atomic lock for the top waiter
   * @pifutex:            the user address of the to futex
   * @hb1:                the from futex hash bucket, must be locked by the caller
   * @hb2:                the to futex hash bucket, must be locked by the caller
   * @key1:               the from futex key
   * @key2:               the to futex key
   * @ps:                 address to store the pi_state pointer
   * @set_waiters:        force setting the FUTEX_WAITERS bit (1) or not (0)
   *
   * Try and get the lock on behalf of the top waiter if we can do it atomically.
   * Wake the top waiter if we succeed.  If the caller specified set_waiters,
   * then direct futex_lock_pi_atomic() to force setting the FUTEX_WAITERS bit.
   * hb1 and hb2 must be held by the caller.
   *
   * Returns:
   *  0 - failed to acquire the lock atomicly
   *  1 - acquired the lock
   * <0 - error
   */
  static int futex_proxy_trylock_atomic(u32 __user *pifutex,
                                   struct futex_hash_bucket *hb1,
                                   struct futex_hash_bucket *hb2,
                                   union futex_key *key1, union futex_key *key2,
                                   struct futex_pi_state **ps, int set_waiters)
  {
          struct futex_q *top_waiter = NULL;
          u32 curval;
          int ret;
  
          if (get_futex_value_locked(&curval, pifutex))
                  return -EFAULT;
  
          /*
           * Find the top_waiter and determine if there are additional waiters.
           * If the caller intends to requeue more than 1 waiter to pifutex,
           * force futex_lock_pi_atomic() to set the FUTEX_WAITERS bit now,
           * as we have means to handle the possible fault.  If not, don't set
           * the bit unecessarily as it will force the subsequent unlock to enter
           * the kernel.
           */
          top_waiter = futex_top_waiter(hb1, key1);
  
          /* There are no waiters, nothing for us to do. */
          if (!top_waiter)
                  return 0;
  
          /* Ensure we requeue to the expected futex. */
          if (!match_futex(top_waiter->requeue_pi_key, key2))
                  return -EINVAL;
  
          /*
           * Try to take the lock for top_waiter.  Set the FUTEX_WAITERS bit in
           * the contended case or if set_waiters is 1.  The pi_state is returned
           * in ps in contended cases.
           */
          ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task,
                                     set_waiters);
          if (ret == 1)
                  requeue_pi_wake_futex(top_waiter, key2, hb2);
  
          return ret;
  }
  
  /**
   * futex_requeue() - Requeue waiters from uaddr1 to uaddr2
   * @uaddr1:     source futex user address
   * @flags:      futex flags (FLAGS_SHARED, etc.)
   * @uaddr2:     target futex user address
   * @nr_wake:    number of waiters to wake (must be 1 for requeue_pi)
   * @nr_requeue: number of waiters to requeue (0-INT_MAX)
   * @cmpval:     @uaddr1 expected value (or %NULL)
   * @requeue_pi: if we are attempting to requeue from a non-pi futex to a
   *              pi futex (pi to pi requeue is not supported)
   *
   * Requeue waiters on uaddr1 to uaddr2. In the requeue_pi case, try to acquire
   * uaddr2 atomically on behalf of the top waiter.
   *
   * Returns:
   * >=0 - on success, the number of tasks requeued or woken
   *  <0 - on error
   */
  static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
                           u32 __user *uaddr2, int nr_wake, int nr_requeue,
                           u32 *cmpval, int requeue_pi)
  {
          union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
          int drop_count = 0, task_count = 0, ret;
          struct futex_pi_state *pi_state = NULL;
          struct futex_hash_bucket *hb1, *hb2;
          struct plist_head *head1;
          struct futex_q *this, *next;
          u32 curval2;
  
          if (requeue_pi) {
                  /*
                   * requeue_pi requires a pi_state, try to allocate it now
                   * without any locks in case it fails.
                   */
                  if (refill_pi_state_cache())
                          return -ENOMEM;
                  /*
                   * requeue_pi must wake as many tasks as it can, up to nr_wake
                   * + nr_requeue, since it acquires the rt_mutex prior to
                   * returning to userspace, so as to not leave the rt_mutex with
                   * waiters and no owner.  However, second and third wake-ups
                   * cannot be predicted as they involve race conditions with the
                   * first wake and a fault while looking up the pi_state.  Both
                   * pthread_cond_signal() and pthread_cond_broadcast() should
                   * use nr_wake=1.
                   */
                  if (nr_wake != 1)
                          return -EINVAL;
          }
  
  retry:
          if (pi_state != NULL) {
                  /*
                   * We will have to lookup the pi_state again, so free this one
                   * to keep the accounting correct.
                   */
                  free_pi_state(pi_state);
                  pi_state = NULL;
          }
  
          ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, VERIFY_READ);
          if (unlikely(ret != 0))
                  goto out;
          ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2,
                              requeue_pi ? VERIFY_WRITE : VERIFY_READ);
          if (unlikely(ret != 0))
                  goto out_put_key1;
  
          hb1 = hash_futex(&key1);
          hb2 = hash_futex(&key2);
  
  retry_private:
          double_lock_hb(hb1, hb2);
  
          if (likely(cmpval != NULL)) {
                  u32 curval;
  
                  ret = get_futex_value_locked(&curval, uaddr1);
  
                  if (unlikely(ret)) {
                          double_unlock_hb(hb1, hb2);
  
                          ret = get_user(curval, uaddr1);
                          if (ret)
                                  goto out_put_keys;
  
                          if (!(flags & FLAGS_SHARED))
                                  goto retry_private;
  
                          put_futex_key(&key2);
                          put_futex_key(&key1);
                          goto retry;
                  }
                  if (curval != *cmpval) {
                          ret = -EAGAIN;
                          goto out_unlock;
                  }
          }
  
          if (requeue_pi && (task_count - nr_wake < nr_requeue)) {
                  /*
                   * Attempt to acquire uaddr2 and wake the top waiter. If we
                   * intend to requeue waiters, force setting the FUTEX_WAITERS
                   * bit.  We force this here where we are able to easily handle
                   * faults rather in the requeue loop below.
                   */
                  ret = futex_proxy_trylock_atomic(uaddr2, hb1, hb2, &key1,
                                                   &key2, &pi_state, nr_requeue);
  
                  /*
                   * At this point the top_waiter has either taken uaddr2 or is
                   * waiting on it.  If the former, then the pi_state will not
                   * exist yet, look it up one more time to ensure we have a
                   * reference to it.
                   */
                  if (ret == 1) {
                          WARN_ON(pi_state);
                          drop_count++;
                          task_count++;
                          ret = get_futex_value_locked(&curval2, uaddr2);
                          if (!ret)
                                  ret = lookup_pi_state(curval2, hb2, &key2,
                                                        &pi_state);
                  }
  
                  switch (ret) {
                  case 0:
                          break;
                  case -EFAULT:
                          double_unlock_hb(hb1, hb2);
                          put_futex_key(&key2);
                          put_futex_key(&key1);
                          ret = fault_in_user_writeable(uaddr2);
                          if (!ret)
                                  goto retry;
                          goto out;
                  case -EAGAIN:
                          /* The owner was exiting, try again. */
                          double_unlock_hb(hb1, hb2);
                          put_futex_key(&key2);
                          put_futex_key(&key1);
                          cond_resched();
                          goto retry;
                  default:
                          goto out_unlock;
                  }
          }
  
          head1 = &hb1->chain;
          plist_for_each_entry_safe(this, next, head1, list) {
                  if (task_count - nr_wake >= nr_requeue)
                          break;
  
                  if (!match_futex(&this->key, &key1))
                          continue;
  
                  /*
                   * FUTEX_WAIT_REQEUE_PI and FUTEX_CMP_REQUEUE_PI should always
                   * be paired with each other and no other futex ops.
                   *
                   * We should never be requeueing a futex_q with a pi_state,
                   * which is awaiting a futex_unlock_pi().
                   */
                  if ((requeue_pi && !this->rt_waiter) ||
                      (!requeue_pi && this->rt_waiter) ||
                      this->pi_state) {
                          ret = -EINVAL;
                          break;
                  }
  
                  /*
                   * Wake nr_wake waiters.  For requeue_pi, if we acquired the
                   * lock, we already woke the top_waiter.  If not, it will be
                   * woken by futex_unlock_pi().
                   */
                  if (++task_count <= nr_wake && !requeue_pi) {
                          wake_futex(this);
                          continue;
                  }
  
                  /* Ensure we requeue to the expected futex for requeue_pi. */
                  if (requeue_pi && !match_futex(this->requeue_pi_key, &key2)) {
                          ret = -EINVAL;
                          break;
                  }
  
                  /*
                   * Requeue nr_requeue waiters and possibly one more in the case
                   * of requeue_pi if we couldn't acquire the lock atomically.
                   */
                  if (requeue_pi) {
                          /* Prepare the waiter to take the rt_mutex. */
                          atomic_inc(&pi_state->refcount);
                          this->pi_state = pi_state;
                          ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
                                                          this->rt_waiter,
                                                          this->task, 1);
                          if (ret == 1) {
                                  /* We got the lock. */
                                  requeue_pi_wake_futex(this, &key2, hb2);
                                  drop_count++;
                                  continue;
                          } else if (ret) {
                                  /* -EDEADLK */
                                  this->pi_state = NULL;
                                  free_pi_state(pi_state);
                                  goto out_unlock;
                          }
                  }
                  requeue_futex(this, hb1, hb2, &key2);
                  drop_count++;
          }
  
  out_unlock:
          double_unlock_hb(hb1, hb2);
  
          /*
           * drop_futex_key_refs() must be called outside the spinlocks. During
           * the requeue we moved futex_q's from the hash bucket at key1 to the
           * one at key2 and updated their key pointer.  We no longer need to
           * hold the references to key1.
           */
          while (--drop_count >= 0)
                  drop_futex_key_refs(&key1);
  
  out_put_keys:
          put_futex_key(&key2);
  out_put_key1:
          put_futex_key(&key1);
  out:
          if (pi_state != NULL)
                  free_pi_state(pi_state);
          return ret ? ret : task_count;
  }
  
  /* The key must be already stored in q->key. */
  static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
          __acquires(&hb->lock)
  {
          struct futex_hash_bucket *hb;
  
          hb = hash_futex(&q->key);
          q->lock_ptr = &hb->lock;
  
          spin_lock(&hb->lock);
          return hb;
  }
  
  static inline void
  queue_unlock(struct futex_q *q, struct futex_hash_bucket *hb)
          __releases(&hb->lock)
  {
          spin_unlock(&hb->lock);
  }
  
  /**
   * queue_me() - Enqueue the futex_q on the futex_hash_bucket
   * @q:  The futex_q to enqueue
   * @hb: The destination hash bucket
   *
   * The hb->lock must be held by the caller, and is released here. A call to
   * queue_me() is typically paired with exactly one call to unqueue_me().  The
   * exceptions involve the PI related operations, which may use unqueue_me_pi()
   * or nothing if the unqueue is done as part of the wake process and the unqueue
   * state is implicit in the state of woken task (see futex_wait_requeue_pi() for
   * an example).
   */
  static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
          __releases(&hb->lock)
  {
          int prio;
  
          /*
           * The priority used to register this element is
           * - either the real thread-priority for the real-time threads
           * (i.e. threads with a priority lower than MAX_RT_PRIO)
           * - or MAX_RT_PRIO for non-RT threads.
           * Thus, all RT-threads are woken first in priority order, and
           * the others are woken last, in FIFO order.
           */
          prio = min(current->normal_prio, MAX_RT_PRIO);
  
          plist_node_init(&q->list, prio);
          plist_add(&q->list, &hb->chain);
          q->task = current;
          spin_unlock(&hb->lock);
  }
  
  /**
   * unqueue_me() - Remove the futex_q from its futex_hash_bucket
   * @q:  The futex_q to unqueue
   *
   * The q->lock_ptr must not be held by the caller. A call to unqueue_me() must
   * be paired with exactly one earlier call to queue_me().
   *
   * Returns:
   *   1 - if the futex_q was still queued (and we removed unqueued it)
   *   0 - if the futex_q was already removed by the waking thread
   */
  static int unqueue_me(struct futex_q *q)
  {
          spinlock_t *lock_ptr;
          int ret = 0;
  
          /* In the common case we don't take the spinlock, which is nice. */
  retry:
          lock_ptr = q->lock_ptr;
          barrier();
          if (lock_ptr != NULL) {
                  spin_lock(lock_ptr);
                  /*
                   * q->lock_ptr can change between reading it and
                   * spin_lock(), causing us to take the wrong lock.  This
                   * corrects the race condition.
                   *
                   * Reasoning goes like this: if we have the wrong lock,
                   * q->lock_ptr must have changed (maybe several times)
                   * between reading it and the spin_lock().  It can
                   * change again after the spin_lock() but only if it was
                   * already changed before the spin_lock().  It cannot,
                   * however, change back to the original value.  Therefore
                   * we can detect whether we acquired the correct lock.
                   */
                  if (unlikely(lock_ptr != q->lock_ptr)) {
                          spin_unlock(lock_ptr);
                          goto retry;
                  }
                  __unqueue_futex(q);
  
                  BUG_ON(q->pi_state);
  
                  spin_unlock(lock_ptr);
                  ret = 1;
          }
  
          drop_futex_key_refs(&q->key);
          return ret;
  }
  
  /*
   * PI futexes can not be requeued and must remove themself from the
   * hash bucket. The hash bucket lock (i.e. lock_ptr) is held on entry
   * and dropped here.
   */
  static void unqueue_me_pi(struct futex_q *q)
          __releases(q->lock_ptr)
  {
          __unqueue_futex(q);
  
          BUG_ON(!q->pi_state);
          free_pi_state(q->pi_state);
          q->pi_state = NULL;
  
          spin_unlock(q->lock_ptr);
  }
  
  /*
   * Fixup the pi_state owner with the new owner.
   *
   * Must be called with hash bucket lock held and mm->sem held for non
   * private futexes.
   */
  static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
                                  struct task_struct *newowner)
  {
          u32 newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
          struct futex_pi_state *pi_state = q->pi_state;
          struct task_struct *oldowner = pi_state->owner;
          u32 uval, uninitialized_var(curval), newval;
          int ret;
  
          /* Owner died? */
          if (!pi_state->owner)
                  newtid |= FUTEX_OWNER_DIED;
  
          /*
           * We are here either because we stole the rtmutex from the
           * previous highest priority waiter or we are the highest priority
           * waiter but failed to get the rtmutex the first time.
           * We have to replace the newowner TID in the user space variable.
           * This must be atomic as we have to preserve the owner died bit here.
           *
           * Note: We write the user space value _before_ changing the pi_state
           * because we can fault here. Imagine swapped out pages or a fork
           * that marked all the anonymous memory readonly for cow.
           *
           * Modifying pi_state _before_ the user space value would
           * leave the pi_state in an inconsistent state when we fault
           * here, because we need to drop the hash bucket lock to
           * handle the fault. This might be observed in the PID check
           * in lookup_pi_state.
           */
  retry:
          if (get_futex_value_locked(&uval, uaddr))
                  goto handle_fault;
  
          while (1) {
                  newval = (uval & FUTEX_OWNER_DIED) | newtid;
  
                  if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval))
                          goto handle_fault;
                  if (curval == uval)
                          break;
                  uval = curval;
          }
  
          /*
           * We fixed up user space. Now we need to fix the pi_state
           * itself.
           */
          if (pi_state->owner != NULL) {
                  raw_spin_lock_irq(&pi_state->owner->pi_lock);
                  WARN_ON(list_empty(&pi_state->list));
                  list_del_init(&pi_state->list);
                  raw_spin_unlock_irq(&pi_state->owner->pi_lock);
          }
  
          pi_state->owner = newowner;
  
          raw_spin_lock_irq(&newowner->pi_lock);
          WARN_ON(!list_empty(&pi_state->list));
          list_add(&pi_state->list, &newowner->pi_state_list);
          raw_spin_unlock_irq(&newowner->pi_lock);
          return 0;
  
          /*
           * To handle the page fault we need to drop the hash bucket
           * lock here. That gives the other task (either the highest priority
           * waiter itself or the task which stole the rtmutex) the
           * chance to try the fixup of the pi_state. So once we are
           * back from handling the fault we need to check the pi_state
           * after reacquiring the hash bucket lock and before trying to
           * do another fixup. When the fixup has been done already we
           * simply return.
           */
  handle_fault:
          spin_unlock(q->lock_ptr);
  
          ret = fault_in_user_writeable(uaddr);
  
          spin_lock(q->lock_ptr);
  
          /*
           * Check if someone else fixed it for us:
           */
          if (pi_state->owner != oldowner)
                  return 0;
  
          if (ret)
                  return ret;
  
          goto retry;
  }
  
  static long futex_wait_restart(struct restart_block *restart);
  
  /**
   * fixup_owner() - Post lock pi_state and corner case management
   * @uaddr:      user address of the futex
   * @q:          futex_q (contains pi_state and access to the rt_mutex)
   * @locked:     if the attempt to take the rt_mutex succeeded (1) or not (0)
   *
   * After attempting to lock an rt_mutex, this function is called to cleanup
   * the pi_state owner as well as handle race conditions that may allow us to
   * acquire the lock. Must be called with the hb lock held.
   *
   * Returns:
   *  1 - success, lock taken
   *  0 - success, lock not taken
   * <0 - on error (-EFAULT)
   */
  static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
  {
          struct task_struct *owner;
          int ret = 0;
  
          if (locked) {
                  /*
                   * Got the lock. We might not be the anticipated owner if we
                   * did a lock-steal - fix up the PI-state in that case:
                   */
                  if (q->pi_state->owner != current)
                          ret = fixup_pi_state_owner(uaddr, q, current);
                  goto out;
          }
  
          /*
           * Catch the rare case, where the lock was released when we were on the
           * way back before we locked the hash bucket.
           */
          if (q->pi_state->owner == current) {
                  /*
                   * Try to get the rt_mutex now. This might fail as some other
                   * task acquired the rt_mutex after we removed ourself from the
                   * rt_mutex waiters list.
                   */
                  if (rt_mutex_trylock(&q->pi_state->pi_mutex)) {
                          locked = 1;
                          goto out;
                  }
  
                  /*
                   * pi_state is incorrect, some other task did a lock steal and
                   * we returned due to timeout or signal without taking the
                   * rt_mutex. Too late.
                   */
                  raw_spin_lock(&q->pi_state->pi_mutex.wait_lock);
                  owner = rt_mutex_owner(&q->pi_state->pi_mutex);
                  if (!owner)
                          owner = rt_mutex_next_owner(&q->pi_state->pi_mutex);
                  raw_spin_unlock(&q->pi_state->pi_mutex.wait_lock);
                  ret = fixup_pi_state_owner(uaddr, q, owner);
                  goto out;
          }
  
          /*
           * Paranoia check. If we did not take the lock, then we should not be
           * the owner of the rt_mutex.
           */
          if (rt_mutex_owner(&q->pi_state->pi_mutex) == current)
                  printk(KERN_ERR "fixup_owner: ret = %d pi-mutex: %p "
                                  "pi-state %p\n", ret,
                                  q->pi_state->pi_mutex.owner,
                                  q->pi_state->owner);
  
  out:
          return ret ? ret : locked;
  }
  
  /**
   * futex_wait_queue_me() - queue_me() and wait for wakeup, timeout, or signal
   * @hb:         the futex hash bucket, must be locked by the caller
   * @q:          the futex_q to queue up on
   * @timeout:    the prepared hrtimer_sleeper, or null for no timeout
   */
  static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q,
                                  struct hrtimer_sleeper *timeout)
  {
          /*
           * The task state is guaranteed to be set before another task can
           * wake it. set_current_state() is implemented using set_mb() and
           * queue_me() calls spin_unlock() upon completion, both serializing
           * access to the hash list and forcing another memory barrier.
           */
          set_current_state(TASK_INTERRUPTIBLE);
          queue_me(q, hb);
  
          /* Arm the timer */
          if (timeout) {
                  hrtimer_start_expires(&timeout->timer, HRTIMER_MODE_ABS);
                  if (!hrtimer_active(&timeout->timer))
                          timeout->task = NULL;
          }
  
          /*
           * If we have been removed from the hash list, then another task
           * has tried to wake us, and we can skip the call to schedule().
           */
          if (likely(!plist_node_empty(&q->list))) {
                  /*
                   * If the timer has already expired, current will already be
                   * flagged for rescheduling. Only call schedule if there
                   * is no timeout, or if it has yet to expire.
                   */
                  if (!timeout || timeout->task)
                          schedule();
          }
          __set_current_state(TASK_RUNNING);
  }
  
  /**
   * futex_wait_setup() - Prepare to wait on a futex
   * @uaddr:      the futex userspace address
   * @val:        the expected value
   * @flags:      futex flags (FLAGS_SHARED, etc.)
   * @q:          the associated futex_q
   * @hb:         storage for hash_bucket pointer to be returned to caller
   *
   * Setup the futex_q and locate the hash_bucket.  Get the futex value and
   * compare it with the expected value.  Handle atomic faults internally.
   * Return with the hb lock held and a q.key reference on success, and unlocked
   * with no q.key reference on failure.
   *
   * Returns:
   *  0 - uaddr contains val and hb has been locked
   * <1 - -EFAULT or -EWOULDBLOCK (uaddr does not contain val) and hb is unlocked
   */
  static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
                             struct futex_q *q, struct futex_hash_bucket **hb)
  {
          u32 uval;
          int ret;
  
          /*
           * Access the page AFTER the hash-bucket is locked.
           * Order is important:
           *
           *   Userspace waiter: val = var; if (cond(val)) futex_wait(&var, val);
           *   Userspace waker:  if (cond(var)) { var = new; futex_wake(&var); }
           *
           * The basic logical guarantee of a futex is that it blocks ONLY
           * if cond(var) is known to be true at the time of blocking, for
           * any cond.  If we locked the hash-bucket after testing *uaddr, that
           * would open a race condition where we could block indefinitely with
           * cond(var) false, which would violate the guarantee.
           *
           * On the other hand, we insert q and release the hash-bucket only
           * after testing *uaddr.  This guarantees that futex_wait() will NOT
           * absorb a wakeup if *uaddr does not match the desired values
           * while the syscall executes.
           */
  retry:
          ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q->key, VERIFY_READ);
          if (unlikely(ret != 0))
                  return ret;
  
  retry_private:
          *hb = queue_lock(q);
  
          ret = get_futex_value_locked(&uval, uaddr);
  
          if (ret) {
                  queue_unlock(q, *hb);
  
                  ret = get_user(uval, uaddr);
                  if (ret)
                          goto out;
  
                  if (!(flags & FLAGS_SHARED))
                          goto retry_private;
  
                  put_futex_key(&q->key);
                  goto retry;
          }
  
          if (uval != val) {
                  queue_unlock(q, *hb);
                  ret = -EWOULDBLOCK;
          }
  
  out:
          if (ret)
                  put_futex_key(&q->key);
          return ret;
  }
  
  static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
                        ktime_t *abs_time, u32 bitset)
  {
          struct hrtimer_sleeper timeout, *to = NULL;
          struct restart_block *restart;
          struct futex_hash_bucket *hb;
          struct futex_q q = futex_q_init;
          int ret;
  
          if (!bitset)
                  return -EINVAL;
          q.bitset = bitset;
  
          if (abs_time) {
                  to = &timeout;
  
                  hrtimer_init_on_stack(&to->timer, (flags & FLAGS_CLOCKRT) ?
                                        CLOCK_REALTIME : CLOCK_MONOTONIC,
                                        HRTIMER_MODE_ABS);
                  hrtimer_init_sleeper(to, current);
                  hrtimer_set_expires_range_ns(&to->timer, *abs_time,
                                               current->timer_slack_ns);
          }
  
  retry:
          /*
           * Prepare to wait on uaddr. On success, holds hb lock and increments
           * q.key refs.
           */
          ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
          if (ret)
                  goto out;
  
          /* queue_me and wait for wakeup, timeout, or a signal. */
          futex_wait_queue_me(hb, &q, to);
  
          /* If we were woken (and unqueued), we succeeded, whatever. */
          ret = 0;
          /* unqueue_me() drops q.key ref */
          if (!unqueue_me(&q))
                  goto out;
          ret = -ETIMEDOUT;
          if (to && !to->task)
                  goto out;
  
          /*
           * We expect signal_pending(current), but we might be the
           * victim of a spurious wakeup as well.
           */
          if (!signal_pending(current))
                  goto retry;
  
          ret = -ERESTARTSYS;
          if (!abs_time)
                  goto out;
  
          restart = &current_thread_info()->restart_block;
          restart->fn = futex_wait_restart;
          restart->futex.uaddr = uaddr;
          restart->futex.val = val;
          restart->futex.time = abs_time->tv64;
          restart->futex.bitset = bitset;
          restart->futex.flags = flags | FLAGS_HAS_TIMEOUT;
  
          ret = -ERESTART_RESTARTBLOCK;
  
  out:
          if (to) {
                  hrtimer_cancel(&to->timer);
                  destroy_hrtimer_on_stack(&to->timer);
          }
          return ret;
  }
  
  
  static long futex_wait_restart(struct restart_block *restart)
  {
          u32 __user *uaddr = restart->futex.uaddr;
          ktime_t t, *tp = NULL;
  
          if (restart->futex.flags & FLAGS_HAS_TIMEOUT) {
                  t.tv64 = restart->futex.time;
                  tp = &t;
          }
          restart->fn = do_no_restart_syscall;
  
          return (long)futex_wait(uaddr, restart->futex.flags,
                                  restart->futex.val, tp, restart->futex.bitset);
  }
  
  
  /*
   * Userspace tried a 0 -> TID atomic transition of the futex value
   * and failed. The kernel side here does the whole locking operation:
   * if there are waiters then it will block, it does PI, etc. (Due to
   * races the kernel might see a 0 value of the futex too.)
   */
  static int futex_lock_pi(u32 __user *uaddr, unsigned int flags, int detect,
                           ktime_t *time, int trylock)
  {
          struct hrtimer_sleeper timeout, *to = NULL;
          struct futex_hash_bucket *hb;
          struct futex_q q = futex_q_init;
          int res, ret;
  
          if (refill_pi_state_cache())
                  return -ENOMEM;
  
          if (time) {
                  to = &timeout;
                  hrtimer_init_on_stack(&to->timer, CLOCK_REALTIME,
                                        HRTIMER_MODE_ABS);
                  hrtimer_init_sleeper(to, current);
                  hrtimer_set_expires(&to->timer, *time);
          }
  
  retry:
          ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q.key, VERIFY_WRITE);
          if (unlikely(ret != 0))
                  goto out;
  
  retry_private:
          hb = queue_lock(&q);
  
          ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current, 0);
          if (unlikely(ret)) {
                  switch (ret) {
                  case 1:
                          /* We got the lock. */
                          ret = 0;
                          goto out_unlock_put_key;
                  case -EFAULT:
                          goto uaddr_faulted;
                  case -EAGAIN:
                          /*
                           * Task is exiting and we just wait for the
                           * exit to complete.
                           */
                          queue_unlock(&q, hb);
                          put_futex_key(&q.key);
                          cond_resched();
                          goto retry;
                  default:
                          goto out_unlock_put_key;
                  }
          }
  
          /*
           * Only actually queue now that the atomic ops are done:
           */
          queue_me(&q, hb);
  
          WARN_ON(!q.pi_state);
          /*
           * Block on the PI mutex:
           */
          if (!trylock)
                  ret = rt_mutex_timed_lock(&q.pi_state->pi_mutex, to, 1);
          else {
                  ret = rt_mutex_trylock(&q.pi_state->pi_mutex);
                  /* Fixup the trylock return value: */
                  ret = ret ? 0 : -EWOULDBLOCK;
          }
  
          spin_lock(q.lock_ptr);
          /*
           * Fixup the pi_state owner and possibly acquire the lock if we
           * haven't already.
           */
          res = fixup_owner(uaddr, &q, !ret);
          /*
           * If fixup_owner() returned an error, proprogate that.  If it acquired
           * the lock, clear our -ETIMEDOUT or -EINTR.
           */
          if (res)
                  ret = (res < 0) ? res : 0;
  
          /*
           * If fixup_owner() faulted and was unable to handle the fault, unlock
           * it and return the fault to userspace.
           */
          if (ret && (rt_mutex_owner(&q.pi_state->pi_mutex) == current))
                  rt_mutex_unlock(&q.pi_state->pi_mutex);
  
          /* Unqueue and drop the lock */
          unqueue_me_pi(&q);
  
          goto out_put_key;
  
  out_unlock_put_key:
          queue_unlock(&q, hb);
  
  out_put_key:
          put_futex_key(&q.key);
  out:
          if (to)
                  destroy_hrtimer_on_stack(&to->timer);
          return ret != -EINTR ? ret : -ERESTARTNOINTR;
  
  uaddr_faulted:
          queue_unlock(&q, hb);
  
          ret = fault_in_user_writeable(uaddr);
          if (ret)
                  goto out_put_key;
  
          if (!(flags & FLAGS_SHARED))
                  goto retry_private;
  
          put_futex_key(&q.key);
          goto retry;
  }
  
  /*
   * Userspace attempted a TID -> 0 atomic transition, and failed.
   * This is the in-kernel slowpath: we look up the PI state (if any),
   * and do the rt-mutex unlock.
   */
  static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
  {
          struct futex_hash_bucket *hb;
          struct futex_q *this, *next;
          struct plist_head *head;
          union futex_key key = FUTEX_KEY_INIT;
          u32 uval, vpid = task_pid_vnr(current);
          int ret;
  
  retry:
          if (get_user(uval, uaddr))
                  return -EFAULT;
          /*
           * We release only a lock we actually own:
           */
          if ((uval & FUTEX_TID_MASK) != vpid)
                  return -EPERM;
  
          ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, VERIFY_WRITE);
          if (unlikely(ret != 0))
                  goto out;
  
          hb = hash_futex(&key);
          spin_lock(&hb->lock);
  
          /*
           * To avoid races, try to do the TID -> 0 atomic transition
           * again. If it succeeds then we can return without waking
           * anyone else up:
           */
          if (!(uval & FUTEX_OWNER_DIED) &&
              cmpxchg_futex_value_locked(&uval, uaddr, vpid, 0))
                  goto pi_faulted;
          /*
           * Rare case: we managed to release the lock atomically,
           * no need to wake anyone else up:
           */
          if (unlikely(uval == vpid))
                  goto out_unlock;
  
          /*
           * Ok, other tasks may need to be woken up - check waiters
           * and do the wakeup if necessary:
           */
          head = &hb->chain;
  
          plist_for_each_entry_safe(this, next, head, list) {
                  if (!match_futex (&this->key, &key))
                          continue;
                  ret = wake_futex_pi(uaddr, uval, this);
                  /*
                   * The atomic access to the futex value
                   * generated a pagefault, so retry the
                   * user-access and the wakeup:
                   */
                  if (ret == -EFAULT)
                          goto pi_faulted;
                  goto out_unlock;
          }
          /*
           * No waiters - kernel unlocks the futex:
           */
          if (!(uval & FUTEX_OWNER_DIED)) {
                  ret = unlock_futex_pi(uaddr, uval);
                  if (ret == -EFAULT)
                          goto pi_faulted;
          }
  
  out_unlock:
          spin_unlock(&hb->lock);
          put_futex_key(&key);
  
  out:
          return ret;
  
  pi_faulted:
          spin_unlock(&hb->lock);
          put_futex_key(&key);
  
          ret = fault_in_user_writeable(uaddr);
          if (!ret)
                  goto retry;
  
          return ret;
  }
  
  /**
   * handle_early_requeue_pi_wakeup() - Detect early wakeup on the initial futex
   * @hb:         the hash_bucket futex_q was original enqueued on
   * @q:          the futex_q woken while waiting to be requeued
   * @key2:       the futex_key of the requeue target futex
   * @timeout:    the timeout associated with the wait (NULL if none)
   *
   * Detect if the task was woken on the initial futex as opposed to the requeue
   * target futex.  If so, determine if it was a timeout or a signal that caused
   * the wakeup and return the appropriate error code to the caller.  Must be
   * called with the hb lock held.
   *
   * Returns
   *  0 - no early wakeup detected
   * <0 - -ETIMEDOUT or -ERESTARTNOINTR
   */
  static inline
  int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb,
                                     struct futex_q *q, union futex_key *key2,
                                     struct hrtimer_sleeper *timeout)
  {
          int ret = 0;
  
          /*
           * With the hb lock held, we avoid races while we process the wakeup.
           * We only need to hold hb (and not hb2) to ensure atomicity as the
           * wakeup code can't change q.key from uaddr to uaddr2 if we hold hb.
           * It can't be requeued from uaddr2 to something else since we don't
           * support a PI aware source futex for requeue.
           */
          if (!match_futex(&q->key, key2)) {
                  WARN_ON(q->lock_ptr && (&hb->lock != q->lock_ptr));
                  /*
                   * We were woken prior to requeue by a timeout or a signal.
                   * Unqueue the futex_q and determine which it was.
                   */
                  plist_del(&q->list, &hb->chain);
  
                  /* Handle spurious wakeups gracefully */
                  ret = -EWOULDBLOCK;
                  if (timeout && !timeout->task)
                          ret = -ETIMEDOUT;
                  else if (signal_pending(current))
                          ret = -ERESTARTNOINTR;
          }
          return ret;
  }
  
  /**
   * futex_wait_requeue_pi() - Wait on uaddr and take uaddr2
   * @uaddr:      the futex we initially wait on (non-pi)
   * @flags:      futex flags (FLAGS_SHARED, FLAGS_CLOCKRT, etc.), they must be
   *              the same type, no requeueing from private to shared, etc.
   * @val:        the expected value of uaddr
   * @abs_time:   absolute timeout
   * @bitset:     32 bit wakeup bitset set by userspace, defaults to all
   * @clockrt:    whether to use CLOCK_REALTIME (1) or CLOCK_MONOTONIC (0)
   * @uaddr2:     the pi futex we will take prior to returning to user-space
   *
   * The caller will wait on uaddr and will be requeued by futex_requeue() to
   * uaddr2 which must be PI aware and unique from uaddr.  Normal wakeup will wake
   * on uaddr2 and complete the acquisition of the rt_mutex prior to returning to
   * userspace.  This ensures the rt_mutex maintains an owner when it has waiters;
   * without one, the pi logic would not know which task to boost/deboost, if
   * there was a need to.
   *
   * We call schedule in futex_wait_queue_me() when we enqueue and return there
   * via the following:
   * 1) wakeup on uaddr2 after an atomic lock acquisition by futex_requeue()
   * 2) wakeup on uaddr2 after a requeue
   * 3) signal
   * 4) timeout
   *
   * If 3, cleanup and return -ERESTARTNOINTR.
   *
   * If 2, we may then block on trying to take the rt_mutex and return via:
   * 5) successful lock
   * 6) signal
   * 7) timeout
   * 8) other lock acquisition failure
   *
   * If 6, return -EWOULDBLOCK (restarting the syscall would do the same).
   *
   * If 4 or 7, we cleanup and return with -ETIMEDOUT.
   *
   * Returns:
   *  0 - On success
   * <0 - On error
   */
  static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
                                   u32 val, ktime_t *abs_time, u32 bitset,
                                   u32 __user *uaddr2)
  {
          struct hrtimer_sleeper timeout, *to = NULL;
          struct rt_mutex_waiter rt_waiter;
          struct rt_mutex *pi_mutex = NULL;
          struct futex_hash_bucket *hb;
          union futex_key key2 = FUTEX_KEY_INIT;
          struct futex_q q = futex_q_init;
          int res, ret;
  
          if (uaddr == uaddr2)
                  return -EINVAL;
  
          if (!bitset)
                  return -EINVAL;
  
          if (abs_time) {
                  to = &timeout;
                  hrtimer_init_on_stack(&to->timer, (flags & FLAGS_CLOCKRT) ?
                                        CLOCK_REALTIME : CLOCK_MONOTONIC,
                                        HRTIMER_MODE_ABS);
                  hrtimer_init_sleeper(to, current);
                  hrtimer_set_expires_range_ns(&to->timer, *abs_time,
                                               current->timer_slack_ns);
          }
  
          /*
           * The waiter is allocated on our stack, manipulated by the requeue
           * code while we sleep on uaddr.
           */
          debug_rt_mutex_init_waiter(&rt_waiter);
          rt_waiter.task = NULL;
  
          ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, VERIFY_WRITE);
          if (unlikely(ret != 0))
                  goto out;
  
          q.bitset = bitset;
          q.rt_waiter = &rt_waiter;
          q.requeue_pi_key = &key2;
  
          /*
           * Prepare to wait on uaddr. On success, increments q.key (key1) ref
           * count.
           */
          ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
          if (ret)
                  goto out_key2;
  
          /* Queue the futex_q, drop the hb lock, wait for wakeup. */
          futex_wait_queue_me(hb, &q, to);
  
          spin_lock(&hb->lock);
          ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
          spin_unlock(&hb->lock);
          if (ret)
                  goto out_put_keys;
  
          /*
           * In order for us to be here, we know our q.key == key2, and since
           * we took the hb->lock above, we also know that futex_requeue() has
           * completed and we no longer have to concern ourselves with a wakeup
           * race with the atomic proxy lock acquisition by the requeue code. The
           * futex_requeue dropped our key1 reference and incremented our key2
           * reference count.
           */
  
          /* Check if the requeue code acquired the second futex for us. */
          if (!q.rt_waiter) {
                  /*
                   * Got the lock. We might not be the anticipated owner if we
                   * did a lock-steal - fix up the PI-state in that case.
                   */
                  if (q.pi_state && (q.pi_state->owner != current)) {
                          spin_lock(q.lock_ptr);
                          ret = fixup_pi_state_owner(uaddr2, &q, current);
                          spin_unlock(q.lock_ptr);
                  }
          } else {
                  /*
                   * We have been woken up by futex_unlock_pi(), a timeout, or a
                   * signal.  futex_unlock_pi() will not destroy the lock_ptr nor
                   * the pi_state.
                   */
                  WARN_ON(!q.pi_state);
                  pi_mutex = &q.pi_state->pi_mutex;
                  ret = rt_mutex_finish_proxy_lock(pi_mutex, to, &rt_waiter, 1);
                  debug_rt_mutex_free_waiter(&rt_waiter);
  
                  spin_lock(q.lock_ptr);
                  /*
                   * Fixup the pi_state owner and possibly acquire the lock if we
                   * haven't already.
                   */
                  res = fixup_owner(uaddr2, &q, !ret);
                  /*
                   * If fixup_owner() returned an error, proprogate that.  If it
                   * acquired the lock, clear -ETIMEDOUT or -EINTR.
                   */
                  if (res)
                          ret = (res < 0) ? res : 0;
  
                  /* Unqueue and drop the lock. */
                  unqueue_me_pi(&q);
          }
  
          /*
           * If fixup_pi_state_owner() faulted and was unable to handle the
           * fault, unlock the rt_mutex and return the fault to userspace.
           */
          if (ret == -EFAULT) {
                  if (pi_mutex && rt_mutex_owner(pi_mutex) == current)
                          rt_mutex_unlock(pi_mutex);
          } else if (ret == -EINTR) {
                  /*
                   * We've already been requeued, but cannot restart by calling
                   * futex_lock_pi() directly. We could restart this syscall, but
                   * it would detect that the user space "val" changed and return
                   * -EWOULDBLOCK.  Save the overhead of the restart and return
                   * -EWOULDBLOCK directly.
                   */
                  ret = -EWOULDBLOCK;
          }
  
  out_put_keys:
          put_futex_key(&q.key);
  out_key2:
          put_futex_key(&key2);
  
  out:
          if (to) {
                  hrtimer_cancel(&to->timer);
                  destroy_hrtimer_on_stack(&to->timer);
          }
          return ret;
  }
  
  /*
   * Support for robust futexes: the kernel cleans up held futexes at
   * thread exit time.
   *
   * Implementation: user-space maintains a per-thread list of locks it
   * is holding. Upon do_exit(), the kernel carefully walks this list,
   * and marks all locks that are owned by this thread with the
   * FUTEX_OWNER_DIED bit, and wakes up a waiter (if any). The list is
   * always manipulated with the lock held, so the list is private and
   * per-thread. Userspace also maintains a per-thread 'list_op_pending'
   * field, to allow the kernel to clean up if the thread dies after
   * acquiring the lock, but just before it could have added itself to
   * the list. There can only be one such pending lock.
   */
  
  /**
   * sys_set_robust_list() - Set the robust-futex list head of a task
   * @head:       pointer to the list-head
   * @len:        length of the list-head, as userspace expects
   */
  SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head,
                  size_t, len)
  {
          if (!futex_cmpxchg_enabled)
                  return -ENOSYS;
          /*
           * The kernel knows only one size for now:
           */
          if (unlikely(len != sizeof(*head)))
                  return -EINVAL;
  
          current->robust_list = head;
  
          return 0;
  }
  
  /**
   * sys_get_robust_list() - Get the robust-futex list head of a task
   * @pid:        pid of the process [zero for current task]
   * @head_ptr:   pointer to a list-head pointer, the kernel fills it in
   * @len_ptr:    pointer to a length field, the kernel fills in the header size
   */
  SYSCALL_DEFINE3(get_robust_list, int, pid,
                  struct robust_list_head __user * __user *, head_ptr,
                  size_t __user *, len_ptr)
  {
          struct robust_list_head __user *head;
          unsigned long ret;
          struct task_struct *p;
  
          if (!futex_cmpxchg_enabled)
                  return -ENOSYS;
  
          WARN_ONCE(1, "deprecated: get_robust_list will be deleted in 2013.\n");
  
          rcu_read_lock();
  
          ret = -ESRCH;
          if (!pid)
                  p = current;
          else {
                  p = find_task_by_vpid(pid);
                  if (!p)
                          goto err_unlock;
          }
  
          ret = -EPERM;
          if (!ptrace_may_access(p, PTRACE_MODE_READ))
                  goto err_unlock;
  
          head = p->robust_list;
          rcu_read_unlock();
  
          if (put_user(sizeof(*head), len_ptr))
                  return -EFAULT;
          return put_user(head, head_ptr);
  
  err_unlock:
          rcu_read_unlock();
  
          return ret;
  }
  
  /*
   * Process a futex-list entry, check whether it's owned by the
   * dying task, and do notification if so:
   */
  int handle_futex_death(u32 __user *uaddr, struct task_struct *curr, int pi)
  {
          u32 uval, uninitialized_var(nval), mval;
  
  retry:
          if (get_user(uval, uaddr))
                  return -1;
  
          if ((uval & FUTEX_TID_MASK) == task_pid_vnr(curr)) {
                  /*
                   * Ok, this dying thread is truly holding a futex
                   * of interest. Set the OWNER_DIED bit atomically
                   * via cmpxchg, and if the value had FUTEX_WAITERS
                   * set, wake up a waiter (if any). (We have to do a
                   * futex_wake() even if OWNER_DIED is already set -
                   * to handle the rare but possible case of recursive
                   * thread-death.) The rest of the cleanup is done in
                   * userspace.
                   */
                  mval = (uval & FUTEX_WAITERS) | FUTEX_OWNER_DIED;
                  /*
                   * We are not holding a lock here, but we want to have
                   * the pagefault_disable/enable() protection because
                   * we want to handle the fault gracefully. If the
                   * access fails we try to fault in the futex with R/W
                   * verification via get_user_pages. get_user() above
                   * does not guarantee R/W access. If that fails we
                   * give up and leave the futex locked.
                   */
                  if (cmpxchg_futex_value_locked(&nval, uaddr, uval, mval)) {
                          if (fault_in_user_writeable(uaddr))
                                  return -1;
                          goto retry;
                  }
                  if (nval != uval)
                          goto retry;
  
                  /*
                   * Wake robust non-PI futexes here. The wakeup of
                   * PI futexes happens in exit_pi_state():
                   */
                  if (!pi && (uval & FUTEX_WAITERS))
                          futex_wake(uaddr, 1, 1, FUTEX_BITSET_MATCH_ANY);
          }
          return 0;
  }
  
  /*
   * Fetch a robust-list pointer. Bit 0 signals PI futexes:
   */
  static inline int fetch_robust_entry(struct robust_list __user **entry,
                                       struct robust_list __user * __user *head,
                                       unsigned int *pi)
  {
          unsigned long uentry;
  
          if (get_user(uentry, (unsigned long __user *)head))
                  return -EFAULT;
  
          *entry = (void __user *)(uentry & ~1UL);
          *pi = uentry & 1;
  
          return 0;
  }
  
  /*
   * Walk curr->robust_list (very carefully, it's a userspace list!)
   * and mark any locks found there dead, and notify any waiters.
   *
   * We silently return on any sign of list-walking problem.
   */
  void exit_robust_list(struct task_struct *curr)
  {
          struct robust_list_head __user *head = curr->robust_list;
          struct robust_list __user *entry, *next_entry, *pending;
          unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
          unsigned int uninitialized_var(next_pi);
          unsigned long futex_offset;
          int rc;
  
          if (!futex_cmpxchg_enabled)
                  return;
  
          /*
           * Fetch the list head (which was registered earlier, via
           * sys_set_robust_list()):
           */
          if (fetch_robust_entry(&entry, &head->list.next, &pi))
                  return;
          /*
           * Fetch the relative futex offset:
           */
          if (get_user(futex_offset, &head->futex_offset))
                  return;
          /*
           * Fetch any possibly pending lock-add first, and handle it
           * if it exists:
           */
          if (fetch_robust_entry(&pending, &head->list_op_pending, &pip))
                  return;
  
          next_entry = NULL;      /* avoid warning with gcc */
          while (entry != &head->list) {
                  /*
                   * Fetch the next entry in the list before calling
                   * handle_futex_death:
                   */
                  rc = fetch_robust_entry(&next_entry, &entry->next, &next_pi);
                  /*
                   * A pending lock might already be on the list, so
                   * don't process it twice:
                   */
                  if (entry != pending)
                          if (handle_futex_death((void __user *)entry + futex_offset,
                                                  curr, pi))
                                  return;
                  if (rc)
                          return;
                  entry = next_entry;
                  pi = next_pi;
                  /*
                   * Avoid excessively long or circular lists:
                   */
                  if (!--limit)
                          break;
  
                  cond_resched();
          }
  
          if (pending)
                  handle_futex_death((void __user *)pending + futex_offset,
                                     curr, pip);
  }
  
  long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
                  u32 __user *uaddr2, u32 val2, u32 val3)
  {
          int cmd = op & FUTEX_CMD_MASK;
          unsigned int flags = 0;
  
          if (!(op & FUTEX_PRIVATE_FLAG))
                  flags |= FLAGS_SHARED;
  
          if (op & FUTEX_CLOCK_REALTIME) {
                  flags |= FLAGS_CLOCKRT;
                  if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI)
                          return -ENOSYS;
          }
  
          switch (cmd) {
          case FUTEX_LOCK_PI:
          case FUTEX_UNLOCK_PI:
          case FUTEX_TRYLOCK_PI:
          case FUTEX_WAIT_REQUEUE_PI:
          case FUTEX_CMP_REQUEUE_PI:
                  if (!futex_cmpxchg_enabled)
                          return -ENOSYS;
          }
  
          switch (cmd) {
          case FUTEX_WAIT:
                  val3 = FUTEX_BITSET_MATCH_ANY;
          case FUTEX_WAIT_BITSET:
                  return futex_wait(uaddr, flags, val, timeout, val3);
          case FUTEX_WAKE:
                  val3 = FUTEX_BITSET_MATCH_ANY;
          case FUTEX_WAKE_BITSET:
                  return futex_wake(uaddr, flags, val, val3);
          case FUTEX_REQUEUE:
                  return futex_requeue(uaddr, flags, uaddr2, val, val2, NULL, 0);
          case FUTEX_CMP_REQUEUE:
                  return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 0);
          case FUTEX_WAKE_OP:
                  return futex_wake_op(uaddr, flags, uaddr2, val, val2, val3);
          case FUTEX_LOCK_PI:
                  return futex_lock_pi(uaddr, flags, val, timeout, 0);
          case FUTEX_UNLOCK_PI:
                  return futex_unlock_pi(uaddr, flags);
          case FUTEX_TRYLOCK_PI:
                  return futex_lock_pi(uaddr, flags, 0, timeout, 1);
          case FUTEX_WAIT_REQUEUE_PI:
                  val3 = FUTEX_BITSET_MATCH_ANY;
                  return futex_wait_requeue_pi(uaddr, flags, val, timeout, val3,
                                               uaddr2);
          case FUTEX_CMP_REQUEUE_PI:
                  return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 1);
          }
          return -ENOSYS;
  }
  
  
  SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
                  struct timespec __user *, utime, u32 __user *, uaddr2,
                  u32, val3)
  {
          struct timespec ts;
          ktime_t t, *tp = NULL;
          u32 val2 = 0;
          int cmd = op & FUTEX_CMD_MASK;
  
          if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI ||
                        cmd == FUTEX_WAIT_BITSET ||
                        cmd == FUTEX_WAIT_REQUEUE_PI)) {
                  if (copy_from_user(&ts, utime, sizeof(ts)) != 0)
                          return -EFAULT;
                  if (!timespec_valid(&ts))
                          return -EINVAL;
  
                  t = timespec_to_ktime(ts);
                  if (cmd == FUTEX_WAIT)
                          t = ktime_add_safe(ktime_get(), t);
                  tp = &t;
          }
          /*
           * requeue parameter in 'utime' if cmd == FUTEX_*_REQUEUE_*.
           * number of waiters to wake in 'utime' if cmd == FUTEX_WAKE_OP.
           */
          if (cmd == FUTEX_REQUEUE || cmd == FUTEX_CMP_REQUEUE ||
              cmd == FUTEX_CMP_REQUEUE_PI || cmd == FUTEX_WAKE_OP)
                  val2 = (u32) (unsigned long) utime;
  
          return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
  }
  
  static int __init futex_init(void)
  {
          u32 curval;
          int i;
  
          /*
           * This will fail and we want it. Some arch implementations do
           * runtime detection of the futex_atomic_cmpxchg_inatomic()
           * functionality. We want to know that before we call in any
           * of the complex code paths. Also we want to prevent
           * registration of robust lists in that case. NULL is
           * guaranteed to fault and we get -EFAULT on functional
           * implementation, the non-functional ones will return
           * -ENOSYS.
           */
          if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT)
                  futex_cmpxchg_enabled = 1;
  
          for (i = 0; i < ARRAY_SIZE(futex_queues); i++) {
                  plist_head_init(&futex_queues[i].chain);
                  spin_lock_init(&futex_queues[i].lock);
          }
  
          return 0;
  }
  __initcall(futex_init);

Cache object: 9f7c9d75f7d3a4026e0f05fed6b6fb74