FreeBSD/Linux Kernel Cross Reference
sys/fs/eventpoll.c

     /*
      *  fs/eventpoll.c (Efficient event retrieval implementation)
      *  Copyright (C) 2001,...,2009  Davide Libenzi
      *
      *  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.
      *
     *  Davide Libenzi <davidel@xmailserver.org>
     *
     */
    
    #include <linux/init.h>
    #include <linux/kernel.h>
    #include <linux/sched.h>
    #include <linux/fs.h>
    #include <linux/file.h>
    #include <linux/signal.h>
    #include <linux/errno.h>
    #include <linux/mm.h>
    #include <linux/slab.h>
    #include <linux/poll.h>
    #include <linux/string.h>
    #include <linux/list.h>
    #include <linux/hash.h>
    #include <linux/spinlock.h>
    #include <linux/syscalls.h>
    #include <linux/rbtree.h>
    #include <linux/wait.h>
    #include <linux/eventpoll.h>
    #include <linux/mount.h>
    #include <linux/bitops.h>
    #include <linux/mutex.h>
    #include <linux/anon_inodes.h>
    #include <linux/device.h>
    #include <asm/uaccess.h>
    #include <asm/io.h>
    #include <asm/mman.h>
    #include <linux/atomic.h>
    #include <linux/proc_fs.h>
    #include <linux/seq_file.h>
    
    /*
     * LOCKING:
     * There are three level of locking required by epoll :
     *
     * 1) epmutex (mutex)
     * 2) ep->mtx (mutex)
     * 3) ep->lock (spinlock)
     *
     * The acquire order is the one listed above, from 1 to 3.
     * We need a spinlock (ep->lock) because we manipulate objects
     * from inside the poll callback, that might be triggered from
     * a wake_up() that in turn might be called from IRQ context.
     * So we can't sleep inside the poll callback and hence we need
     * a spinlock. During the event transfer loop (from kernel to
     * user space) we could end up sleeping due a copy_to_user(), so
     * we need a lock that will allow us to sleep. This lock is a
     * mutex (ep->mtx). It is acquired during the event transfer loop,
     * during epoll_ctl(EPOLL_CTL_DEL) and during eventpoll_release_file().
     * Then we also need a global mutex to serialize eventpoll_release_file()
     * and ep_free().
     * This mutex is acquired by ep_free() during the epoll file
     * cleanup path and it is also acquired by eventpoll_release_file()
     * if a file has been pushed inside an epoll set and it is then
     * close()d without a previous call to epoll_ctl(EPOLL_CTL_DEL).
     * It is also acquired when inserting an epoll fd onto another epoll
     * fd. We do this so that we walk the epoll tree and ensure that this
     * insertion does not create a cycle of epoll file descriptors, which
     * could lead to deadlock. We need a global mutex to prevent two
     * simultaneous inserts (A into B and B into A) from racing and
     * constructing a cycle without either insert observing that it is
     * going to.
     * It is necessary to acquire multiple "ep->mtx"es at once in the
     * case when one epoll fd is added to another. In this case, we
     * always acquire the locks in the order of nesting (i.e. after
     * epoll_ctl(e1, EPOLL_CTL_ADD, e2), e1->mtx will always be acquired
     * before e2->mtx). Since we disallow cycles of epoll file
     * descriptors, this ensures that the mutexes are well-ordered. In
     * order to communicate this nesting to lockdep, when walking a tree
     * of epoll file descriptors, we use the current recursion depth as
     * the lockdep subkey.
     * It is possible to drop the "ep->mtx" and to use the global
     * mutex "epmutex" (together with "ep->lock") to have it working,
     * but having "ep->mtx" will make the interface more scalable.
     * Events that require holding "epmutex" are very rare, while for
     * normal operations the epoll private "ep->mtx" will guarantee
     * a better scalability.
     */
    
    /* Epoll private bits inside the event mask */
    #define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET)
    
    /* Maximum number of nesting allowed inside epoll sets */
    #define EP_MAX_NESTS 4
    
    #define EP_MAX_EVENTS (INT_MAX / sizeof(struct epoll_event))
    
   #define EP_UNACTIVE_PTR ((void *) -1L)
   
   #define EP_ITEM_COST (sizeof(struct epitem) + sizeof(struct eppoll_entry))
   
   struct epoll_filefd {
           struct file *file;
           int fd;
   };
   
   /*
    * Structure used to track possible nested calls, for too deep recursions
    * and loop cycles.
    */
   struct nested_call_node {
           struct list_head llink;
           void *cookie;
           void *ctx;
   };
   
   /*
    * This structure is used as collector for nested calls, to check for
    * maximum recursion dept and loop cycles.
    */
   struct nested_calls {
           struct list_head tasks_call_list;
           spinlock_t lock;
   };
   
   /*
    * Each file descriptor added to the eventpoll interface will
    * have an entry of this type linked to the "rbr" RB tree.
    */
   struct epitem {
           /* RB tree node used to link this structure to the eventpoll RB tree */
           struct rb_node rbn;
   
           /* List header used to link this structure to the eventpoll ready list */
           struct list_head rdllink;
   
           /*
            * Works together "struct eventpoll"->ovflist in keeping the
            * single linked chain of items.
            */
           struct epitem *next;
   
           /* The file descriptor information this item refers to */
           struct epoll_filefd ffd;
   
           /* Number of active wait queue attached to poll operations */
           int nwait;
   
           /* List containing poll wait queues */
           struct list_head pwqlist;
   
           /* The "container" of this item */
           struct eventpoll *ep;
   
           /* List header used to link this item to the "struct file" items list */
           struct list_head fllink;
   
           /* wakeup_source used when EPOLLWAKEUP is set */
           struct wakeup_source *ws;
   
           /* The structure that describe the interested events and the source fd */
           struct epoll_event event;
   };
   
   /*
    * This structure is stored inside the "private_data" member of the file
    * structure and represents the main data structure for the eventpoll
    * interface.
    */
   struct eventpoll {
           /* Protect the access to this structure */
           spinlock_t lock;
   
           /*
            * This mutex is used to ensure that files are not removed
            * while epoll is using them. This is held during the event
            * collection loop, the file cleanup path, the epoll file exit
            * code and the ctl operations.
            */
           struct mutex mtx;
   
           /* Wait queue used by sys_epoll_wait() */
           wait_queue_head_t wq;
   
           /* Wait queue used by file->poll() */
           wait_queue_head_t poll_wait;
   
           /* List of ready file descriptors */
           struct list_head rdllist;
   
           /* RB tree root used to store monitored fd structs */
           struct rb_root rbr;
   
           /*
            * This is a single linked list that chains all the "struct epitem" that
            * happened while transferring ready events to userspace w/out
            * holding ->lock.
            */
           struct epitem *ovflist;
   
           /* wakeup_source used when ep_scan_ready_list is running */
           struct wakeup_source *ws;
   
           /* The user that created the eventpoll descriptor */
           struct user_struct *user;
   
           struct file *file;
   
           /* used to optimize loop detection check */
           int visited;
           struct list_head visited_list_link;
   };
   
   /* Wait structure used by the poll hooks */
   struct eppoll_entry {
           /* List header used to link this structure to the "struct epitem" */
           struct list_head llink;
   
           /* The "base" pointer is set to the container "struct epitem" */
           struct epitem *base;
   
           /*
            * Wait queue item that will be linked to the target file wait
            * queue head.
            */
           wait_queue_t wait;
   
           /* The wait queue head that linked the "wait" wait queue item */
           wait_queue_head_t *whead;
   };
   
   /* Wrapper struct used by poll queueing */
   struct ep_pqueue {
           poll_table pt;
           struct epitem *epi;
   };
   
   /* Used by the ep_send_events() function as callback private data */
   struct ep_send_events_data {
           int maxevents;
           struct epoll_event __user *events;
   };
   
   /*
    * Configuration options available inside /proc/sys/fs/epoll/
    */
   /* Maximum number of epoll watched descriptors, per user */
   static long max_user_watches __read_mostly;
   
   /*
    * This mutex is used to serialize ep_free() and eventpoll_release_file().
    */
   static DEFINE_MUTEX(epmutex);
   
   /* Used to check for epoll file descriptor inclusion loops */
   static struct nested_calls poll_loop_ncalls;
   
   /* Used for safe wake up implementation */
   static struct nested_calls poll_safewake_ncalls;
   
   /* Used to call file's f_op->poll() under the nested calls boundaries */
   static struct nested_calls poll_readywalk_ncalls;
   
   /* Slab cache used to allocate "struct epitem" */
   static struct kmem_cache *epi_cache __read_mostly;
   
   /* Slab cache used to allocate "struct eppoll_entry" */
   static struct kmem_cache *pwq_cache __read_mostly;
   
   /* Visited nodes during ep_loop_check(), so we can unset them when we finish */
   static LIST_HEAD(visited_list);
   
   /*
    * List of files with newly added links, where we may need to limit the number
    * of emanating paths. Protected by the epmutex.
    */
   static LIST_HEAD(tfile_check_list);
   
   #ifdef CONFIG_SYSCTL
   
   #include <linux/sysctl.h>
   
   static long zero;
   static long long_max = LONG_MAX;
   
   ctl_table epoll_table[] = {
           {
                   .procname       = "max_user_watches",
                   .data           = &max_user_watches,
                   .maxlen         = sizeof(max_user_watches),
                   .mode           = 0644,
                   .proc_handler   = proc_doulongvec_minmax,
                   .extra1         = &zero,
                   .extra2         = &long_max,
           },
           { }
   };
   #endif /* CONFIG_SYSCTL */
   
   static const struct file_operations eventpoll_fops;
   
   static inline int is_file_epoll(struct file *f)
   {
           return f->f_op == &eventpoll_fops;
   }
   
   /* Setup the structure that is used as key for the RB tree */
   static inline void ep_set_ffd(struct epoll_filefd *ffd,
                                 struct file *file, int fd)
   {
           ffd->file = file;
           ffd->fd = fd;
   }
   
   /* Compare RB tree keys */
   static inline int ep_cmp_ffd(struct epoll_filefd *p1,
                                struct epoll_filefd *p2)
   {
           return (p1->file > p2->file ? +1:
                   (p1->file < p2->file ? -1 : p1->fd - p2->fd));
   }
   
   /* Tells us if the item is currently linked */
   static inline int ep_is_linked(struct list_head *p)
   {
           return !list_empty(p);
   }
   
   static inline struct eppoll_entry *ep_pwq_from_wait(wait_queue_t *p)
   {
           return container_of(p, struct eppoll_entry, wait);
   }
   
   /* Get the "struct epitem" from a wait queue pointer */
   static inline struct epitem *ep_item_from_wait(wait_queue_t *p)
   {
           return container_of(p, struct eppoll_entry, wait)->base;
   }
   
   /* Get the "struct epitem" from an epoll queue wrapper */
   static inline struct epitem *ep_item_from_epqueue(poll_table *p)
   {
           return container_of(p, struct ep_pqueue, pt)->epi;
   }
   
   /* Tells if the epoll_ctl(2) operation needs an event copy from userspace */
   static inline int ep_op_has_event(int op)
   {
           return op != EPOLL_CTL_DEL;
   }
   
   /* Initialize the poll safe wake up structure */
   static void ep_nested_calls_init(struct nested_calls *ncalls)
   {
           INIT_LIST_HEAD(&ncalls->tasks_call_list);
           spin_lock_init(&ncalls->lock);
   }
   
   /**
    * ep_events_available - Checks if ready events might be available.
    *
    * @ep: Pointer to the eventpoll context.
    *
    * Returns: Returns a value different than zero if ready events are available,
    *          or zero otherwise.
    */
   static inline int ep_events_available(struct eventpoll *ep)
   {
           return !list_empty(&ep->rdllist) || ep->ovflist != EP_UNACTIVE_PTR;
   }
   
   /**
    * ep_call_nested - Perform a bound (possibly) nested call, by checking
    *                  that the recursion limit is not exceeded, and that
    *                  the same nested call (by the meaning of same cookie) is
    *                  no re-entered.
    *
    * @ncalls: Pointer to the nested_calls structure to be used for this call.
    * @max_nests: Maximum number of allowed nesting calls.
    * @nproc: Nested call core function pointer.
    * @priv: Opaque data to be passed to the @nproc callback.
    * @cookie: Cookie to be used to identify this nested call.
    * @ctx: This instance context.
    *
    * Returns: Returns the code returned by the @nproc callback, or -1 if
    *          the maximum recursion limit has been exceeded.
    */
   static int ep_call_nested(struct nested_calls *ncalls, int max_nests,
                             int (*nproc)(void *, void *, int), void *priv,
                             void *cookie, void *ctx)
   {
           int error, call_nests = 0;
           unsigned long flags;
           struct list_head *lsthead = &ncalls->tasks_call_list;
           struct nested_call_node *tncur;
           struct nested_call_node tnode;
   
           spin_lock_irqsave(&ncalls->lock, flags);
   
           /*
            * Try to see if the current task is already inside this wakeup call.
            * We use a list here, since the population inside this set is always
            * very much limited.
            */
           list_for_each_entry(tncur, lsthead, llink) {
                   if (tncur->ctx == ctx &&
                       (tncur->cookie == cookie || ++call_nests > max_nests)) {
                           /*
                            * Ops ... loop detected or maximum nest level reached.
                            * We abort this wake by breaking the cycle itself.
                            */
                           error = -1;
                           goto out_unlock;
                   }
           }
   
           /* Add the current task and cookie to the list */
           tnode.ctx = ctx;
           tnode.cookie = cookie;
           list_add(&tnode.llink, lsthead);
   
           spin_unlock_irqrestore(&ncalls->lock, flags);
   
           /* Call the nested function */
           error = (*nproc)(priv, cookie, call_nests);
   
           /* Remove the current task from the list */
           spin_lock_irqsave(&ncalls->lock, flags);
           list_del(&tnode.llink);
   out_unlock:
           spin_unlock_irqrestore(&ncalls->lock, flags);
   
           return error;
   }
   
   /*
    * As described in commit 0ccf831cb lockdep: annotate epoll
    * the use of wait queues used by epoll is done in a very controlled
    * manner. Wake ups can nest inside each other, but are never done
    * with the same locking. For example:
    *
    *   dfd = socket(...);
    *   efd1 = epoll_create();
    *   efd2 = epoll_create();
    *   epoll_ctl(efd1, EPOLL_CTL_ADD, dfd, ...);
    *   epoll_ctl(efd2, EPOLL_CTL_ADD, efd1, ...);
    *
    * When a packet arrives to the device underneath "dfd", the net code will
    * issue a wake_up() on its poll wake list. Epoll (efd1) has installed a
    * callback wakeup entry on that queue, and the wake_up() performed by the
    * "dfd" net code will end up in ep_poll_callback(). At this point epoll
    * (efd1) notices that it may have some event ready, so it needs to wake up
    * the waiters on its poll wait list (efd2). So it calls ep_poll_safewake()
    * that ends up in another wake_up(), after having checked about the
    * recursion constraints. That are, no more than EP_MAX_POLLWAKE_NESTS, to
    * avoid stack blasting.
    *
    * When CONFIG_DEBUG_LOCK_ALLOC is enabled, make sure lockdep can handle
    * this special case of epoll.
    */
   #ifdef CONFIG_DEBUG_LOCK_ALLOC
   static inline void ep_wake_up_nested(wait_queue_head_t *wqueue,
                                        unsigned long events, int subclass)
   {
           unsigned long flags;
   
           spin_lock_irqsave_nested(&wqueue->lock, flags, subclass);
           wake_up_locked_poll(wqueue, events);
           spin_unlock_irqrestore(&wqueue->lock, flags);
   }
   #else
   static inline void ep_wake_up_nested(wait_queue_head_t *wqueue,
                                        unsigned long events, int subclass)
   {
           wake_up_poll(wqueue, events);
   }
   #endif
   
   static int ep_poll_wakeup_proc(void *priv, void *cookie, int call_nests)
   {
           ep_wake_up_nested((wait_queue_head_t *) cookie, POLLIN,
                             1 + call_nests);
           return 0;
   }
   
   /*
    * Perform a safe wake up of the poll wait list. The problem is that
    * with the new callback'd wake up system, it is possible that the
    * poll callback is reentered from inside the call to wake_up() done
    * on the poll wait queue head. The rule is that we cannot reenter the
    * wake up code from the same task more than EP_MAX_NESTS times,
    * and we cannot reenter the same wait queue head at all. This will
    * enable to have a hierarchy of epoll file descriptor of no more than
    * EP_MAX_NESTS deep.
    */
   static void ep_poll_safewake(wait_queue_head_t *wq)
   {
           int this_cpu = get_cpu();
   
           ep_call_nested(&poll_safewake_ncalls, EP_MAX_NESTS,
                          ep_poll_wakeup_proc, NULL, wq, (void *) (long) this_cpu);
   
           put_cpu();
   }
   
   static void ep_remove_wait_queue(struct eppoll_entry *pwq)
   {
           wait_queue_head_t *whead;
   
           rcu_read_lock();
           /* If it is cleared by POLLFREE, it should be rcu-safe */
           whead = rcu_dereference(pwq->whead);
           if (whead)
                   remove_wait_queue(whead, &pwq->wait);
           rcu_read_unlock();
   }
   
   /*
    * This function unregisters poll callbacks from the associated file
    * descriptor.  Must be called with "mtx" held (or "epmutex" if called from
    * ep_free).
    */
   static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi)
   {
           struct list_head *lsthead = &epi->pwqlist;
           struct eppoll_entry *pwq;
   
           while (!list_empty(lsthead)) {
                   pwq = list_first_entry(lsthead, struct eppoll_entry, llink);
   
                   list_del(&pwq->llink);
                   ep_remove_wait_queue(pwq);
                   kmem_cache_free(pwq_cache, pwq);
           }
   }
   
   /**
    * ep_scan_ready_list - Scans the ready list in a way that makes possible for
    *                      the scan code, to call f_op->poll(). Also allows for
    *                      O(NumReady) performance.
    *
    * @ep: Pointer to the epoll private data structure.
    * @sproc: Pointer to the scan callback.
    * @priv: Private opaque data passed to the @sproc callback.
    * @depth: The current depth of recursive f_op->poll calls.
    *
    * Returns: The same integer error code returned by the @sproc callback.
    */
   static int ep_scan_ready_list(struct eventpoll *ep,
                                 int (*sproc)(struct eventpoll *,
                                              struct list_head *, void *),
                                 void *priv,
                                 int depth)
   {
           int error, pwake = 0;
           unsigned long flags;
           struct epitem *epi, *nepi;
           LIST_HEAD(txlist);
   
           /*
            * We need to lock this because we could be hit by
            * eventpoll_release_file() and epoll_ctl().
            */
           mutex_lock_nested(&ep->mtx, depth);
   
           /*
            * Steal the ready list, and re-init the original one to the
            * empty list. Also, set ep->ovflist to NULL so that events
            * happening while looping w/out locks, are not lost. We cannot
            * have the poll callback to queue directly on ep->rdllist,
            * because we want the "sproc" callback to be able to do it
            * in a lockless way.
            */
           spin_lock_irqsave(&ep->lock, flags);
           list_splice_init(&ep->rdllist, &txlist);
           ep->ovflist = NULL;
           spin_unlock_irqrestore(&ep->lock, flags);
   
           /*
            * Now call the callback function.
            */
           error = (*sproc)(ep, &txlist, priv);
   
           spin_lock_irqsave(&ep->lock, flags);
           /*
            * During the time we spent inside the "sproc" callback, some
            * other events might have been queued by the poll callback.
            * We re-insert them inside the main ready-list here.
            */
           for (nepi = ep->ovflist; (epi = nepi) != NULL;
                nepi = epi->next, epi->next = EP_UNACTIVE_PTR) {
                   /*
                    * We need to check if the item is already in the list.
                    * During the "sproc" callback execution time, items are
                    * queued into ->ovflist but the "txlist" might already
                    * contain them, and the list_splice() below takes care of them.
                    */
                   if (!ep_is_linked(&epi->rdllink)) {
                           list_add_tail(&epi->rdllink, &ep->rdllist);
                           __pm_stay_awake(epi->ws);
                   }
           }
           /*
            * We need to set back ep->ovflist to EP_UNACTIVE_PTR, so that after
            * releasing the lock, events will be queued in the normal way inside
            * ep->rdllist.
            */
           ep->ovflist = EP_UNACTIVE_PTR;
   
           /*
            * Quickly re-inject items left on "txlist".
            */
           list_splice(&txlist, &ep->rdllist);
           __pm_relax(ep->ws);
   
           if (!list_empty(&ep->rdllist)) {
                   /*
                    * Wake up (if active) both the eventpoll wait list and
                    * the ->poll() wait list (delayed after we release the lock).
                    */
                   if (waitqueue_active(&ep->wq))
                           wake_up_locked(&ep->wq);
                   if (waitqueue_active(&ep->poll_wait))
                           pwake++;
           }
           spin_unlock_irqrestore(&ep->lock, flags);
   
           mutex_unlock(&ep->mtx);
   
           /* We have to call this outside the lock */
           if (pwake)
                   ep_poll_safewake(&ep->poll_wait);
   
           return error;
   }
   
   /*
    * Removes a "struct epitem" from the eventpoll RB tree and deallocates
    * all the associated resources. Must be called with "mtx" held.
    */
   static int ep_remove(struct eventpoll *ep, struct epitem *epi)
   {
           unsigned long flags;
           struct file *file = epi->ffd.file;
   
           /*
            * Removes poll wait queue hooks. We _have_ to do this without holding
            * the "ep->lock" otherwise a deadlock might occur. This because of the
            * sequence of the lock acquisition. Here we do "ep->lock" then the wait
            * queue head lock when unregistering the wait queue. The wakeup callback
            * will run by holding the wait queue head lock and will call our callback
            * that will try to get "ep->lock".
            */
           ep_unregister_pollwait(ep, epi);
   
           /* Remove the current item from the list of epoll hooks */
           spin_lock(&file->f_lock);
           if (ep_is_linked(&epi->fllink))
                   list_del_init(&epi->fllink);
           spin_unlock(&file->f_lock);
   
           rb_erase(&epi->rbn, &ep->rbr);
   
           spin_lock_irqsave(&ep->lock, flags);
           if (ep_is_linked(&epi->rdllink))
                   list_del_init(&epi->rdllink);
           spin_unlock_irqrestore(&ep->lock, flags);
   
           wakeup_source_unregister(epi->ws);
   
           /* At this point it is safe to free the eventpoll item */
           kmem_cache_free(epi_cache, epi);
   
           atomic_long_dec(&ep->user->epoll_watches);
   
           return 0;
   }
   
   static void ep_free(struct eventpoll *ep)
   {
           struct rb_node *rbp;
           struct epitem *epi;
   
           /* We need to release all tasks waiting for these file */
           if (waitqueue_active(&ep->poll_wait))
                   ep_poll_safewake(&ep->poll_wait);
   
           /*
            * We need to lock this because we could be hit by
            * eventpoll_release_file() while we're freeing the "struct eventpoll".
            * We do not need to hold "ep->mtx" here because the epoll file
            * is on the way to be removed and no one has references to it
            * anymore. The only hit might come from eventpoll_release_file() but
            * holding "epmutex" is sufficient here.
            */
           mutex_lock(&epmutex);
   
           /*
            * Walks through the whole tree by unregistering poll callbacks.
            */
           for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
                   epi = rb_entry(rbp, struct epitem, rbn);
   
                   ep_unregister_pollwait(ep, epi);
           }
   
           /*
            * Walks through the whole tree by freeing each "struct epitem". At this
            * point we are sure no poll callbacks will be lingering around, and also by
            * holding "epmutex" we can be sure that no file cleanup code will hit
            * us during this operation. So we can avoid the lock on "ep->lock".
            */
           while ((rbp = rb_first(&ep->rbr)) != NULL) {
                   epi = rb_entry(rbp, struct epitem, rbn);
                   ep_remove(ep, epi);
           }
   
           mutex_unlock(&epmutex);
           mutex_destroy(&ep->mtx);
           free_uid(ep->user);
           wakeup_source_unregister(ep->ws);
           kfree(ep);
   }
   
   static int ep_eventpoll_release(struct inode *inode, struct file *file)
   {
           struct eventpoll *ep = file->private_data;
   
           if (ep)
                   ep_free(ep);
   
           return 0;
   }
   
   static int ep_read_events_proc(struct eventpoll *ep, struct list_head *head,
                                  void *priv)
   {
           struct epitem *epi, *tmp;
           poll_table pt;
   
           init_poll_funcptr(&pt, NULL);
           list_for_each_entry_safe(epi, tmp, head, rdllink) {
                   pt._key = epi->event.events;
                   if (epi->ffd.file->f_op->poll(epi->ffd.file, &pt) &
                       epi->event.events)
                           return POLLIN | POLLRDNORM;
                   else {
                           /*
                            * Item has been dropped into the ready list by the poll
                            * callback, but it's not actually ready, as far as
                            * caller requested events goes. We can remove it here.
                            */
                           __pm_relax(epi->ws);
                           list_del_init(&epi->rdllink);
                   }
           }
   
           return 0;
   }
   
   static int ep_poll_readyevents_proc(void *priv, void *cookie, int call_nests)
   {
           return ep_scan_ready_list(priv, ep_read_events_proc, NULL, call_nests + 1);
   }
   
   static unsigned int ep_eventpoll_poll(struct file *file, poll_table *wait)
   {
           int pollflags;
           struct eventpoll *ep = file->private_data;
   
           /* Insert inside our poll wait queue */
           poll_wait(file, &ep->poll_wait, wait);
   
           /*
            * Proceed to find out if wanted events are really available inside
            * the ready list. This need to be done under ep_call_nested()
            * supervision, since the call to f_op->poll() done on listed files
            * could re-enter here.
            */
           pollflags = ep_call_nested(&poll_readywalk_ncalls, EP_MAX_NESTS,
                                      ep_poll_readyevents_proc, ep, ep, current);
   
           return pollflags != -1 ? pollflags : 0;
   }
   
   #ifdef CONFIG_PROC_FS
   static int ep_show_fdinfo(struct seq_file *m, struct file *f)
   {
           struct eventpoll *ep = f->private_data;
           struct rb_node *rbp;
           int ret = 0;
   
           mutex_lock(&ep->mtx);
           for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
                   struct epitem *epi = rb_entry(rbp, struct epitem, rbn);
   
                   ret = seq_printf(m, "tfd: %8d events: %8x data: %16llx\n",
                                    epi->ffd.fd, epi->event.events,
                                    (long long)epi->event.data);
                   if (ret)
                           break;
           }
           mutex_unlock(&ep->mtx);
   
           return ret;
   }
   #endif
   
   /* File callbacks that implement the eventpoll file behaviour */
   static const struct file_operations eventpoll_fops = {
   #ifdef CONFIG_PROC_FS
           .show_fdinfo    = ep_show_fdinfo,
   #endif
           .release        = ep_eventpoll_release,
           .poll           = ep_eventpoll_poll,
           .llseek         = noop_llseek,
   };
   
   /*
    * This is called from eventpoll_release() to unlink files from the eventpoll
    * interface. We need to have this facility to cleanup correctly files that are
    * closed without being removed from the eventpoll interface.
    */
   void eventpoll_release_file(struct file *file)
   {
           struct list_head *lsthead = &file->f_ep_links;
           struct eventpoll *ep;
           struct epitem *epi;
   
           /*
            * We don't want to get "file->f_lock" because it is not
            * necessary. It is not necessary because we're in the "struct file"
            * cleanup path, and this means that no one is using this file anymore.
            * So, for example, epoll_ctl() cannot hit here since if we reach this
            * point, the file counter already went to zero and fget() would fail.
            * The only hit might come from ep_free() but by holding the mutex
            * will correctly serialize the operation. We do need to acquire
            * "ep->mtx" after "epmutex" because ep_remove() requires it when called
            * from anywhere but ep_free().
            *
            * Besides, ep_remove() acquires the lock, so we can't hold it here.
            */
           mutex_lock(&epmutex);
   
           while (!list_empty(lsthead)) {
                   epi = list_first_entry(lsthead, struct epitem, fllink);
   
                   ep = epi->ep;
                   list_del_init(&epi->fllink);
                   mutex_lock_nested(&ep->mtx, 0);
                   ep_remove(ep, epi);
                   mutex_unlock(&ep->mtx);
           }
   
           mutex_unlock(&epmutex);
   }
   
   static int ep_alloc(struct eventpoll **pep)
   {
           int error;
           struct user_struct *user;
           struct eventpoll *ep;
   
           user = get_current_user();
           error = -ENOMEM;
           ep = kzalloc(sizeof(*ep), GFP_KERNEL);
           if (unlikely(!ep))
                   goto free_uid;
   
           spin_lock_init(&ep->lock);
           mutex_init(&ep->mtx);
           init_waitqueue_head(&ep->wq);
           init_waitqueue_head(&ep->poll_wait);
           INIT_LIST_HEAD(&ep->rdllist);
           ep->rbr = RB_ROOT;
           ep->ovflist = EP_UNACTIVE_PTR;
           ep->user = user;
   
           *pep = ep;
   
           return 0;
   
   free_uid:
           free_uid(user);
           return error;
   }
   
   /*
    * Search the file inside the eventpoll tree. The RB tree operations
    * are protected by the "mtx" mutex, and ep_find() must be called with
    * "mtx" held.
    */
   static struct epitem *ep_find(struct eventpoll *ep, struct file *file, int fd)
   {
           int kcmp;
           struct rb_node *rbp;
           struct epitem *epi, *epir = NULL;
           struct epoll_filefd ffd;
   
           ep_set_ffd(&ffd, file, fd);
           for (rbp = ep->rbr.rb_node; rbp; ) {
                   epi = rb_entry(rbp, struct epitem, rbn);
                   kcmp = ep_cmp_ffd(&ffd, &epi->ffd);
                   if (kcmp > 0)
                           rbp = rbp->rb_right;
                   else if (kcmp < 0)
                           rbp = rbp->rb_left;
                   else {
                           epir = epi;
                           break;
                   }
           }
   
           return epir;
   }
   
   /*
    * This is the callback that is passed to the wait queue wakeup
    * mechanism. It is called by the stored file descriptors when they
    * have events to report.
    */
   static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *key)
   {
           int pwake = 0;
           unsigned long flags;
           struct epitem *epi = ep_item_from_wait(wait);
           struct eventpoll *ep = epi->ep;
   
           if ((unsigned long)key & POLLFREE) {
                   ep_pwq_from_wait(wait)->whead = NULL;
                   /*
                    * whead = NULL above can race with ep_remove_wait_queue()
                    * which can do another remove_wait_queue() after us, so we
                    * can't use __remove_wait_queue(). whead->lock is held by
                    * the caller.
                    */
                   list_del_init(&wait->task_list);
           }
   
           spin_lock_irqsave(&ep->lock, flags);
   
           /*
            * If the event mask does not contain any poll(2) event, we consider the
            * descriptor to be disabled. This condition is likely the effect of the
            * EPOLLONESHOT bit that disables the descriptor when an event is received,
            * until the next EPOLL_CTL_MOD will be issued.
            */
           if (!(epi->event.events & ~EP_PRIVATE_BITS))
                   goto out_unlock;
   
           /*
            * Check the events coming with the callback. At this stage, not
            * every device reports the events in the "key" parameter of the
            * callback. We need to be able to handle both cases here, hence the
            * test for "key" != NULL before the event match test.
            */
           if (key && !((unsigned long) key & epi->event.events))
                   goto out_unlock;
   
           /*
            * If we are transferring events to userspace, we can hold no locks
            * (because we're accessing user memory, and because of linux f_op->poll()
            * semantics). All the events that happen during that period of time are
            * chained in ep->ovflist and requeued later on.
            */
           if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
                   if (epi->next == EP_UNACTIVE_PTR) {
                           epi->next = ep->ovflist;
                           ep->ovflist = epi;
                           if (epi->ws) {
                                   /*
                                    * Activate ep->ws since epi->ws may get
                                    * deactivated at any time.
                                    */
                                   __pm_stay_awake(ep->ws);
                           }
   
                   }
                   goto out_unlock;
           }
   
           /* If this file is already in the ready list we exit soon */
           if (!ep_is_linked(&epi->rdllink)) {
                   list_add_tail(&epi->rdllink, &ep->rdllist);
                   __pm_stay_awake(epi->ws);
           }
   
           /*
            * Wake up ( if active ) both the eventpoll wait list and the ->poll()
            * wait list.
            */
           if (waitqueue_active(&ep->wq))
                   wake_up_locked(&ep->wq);
           if (waitqueue_active(&ep->poll_wait))
                   pwake++;
   
   out_unlock:
          spin_unlock_irqrestore(&ep->lock, flags);
  
          /* We have to call this outside the lock */
          if (pwake)
                  ep_poll_safewake(&ep->poll_wait);
  
          return 1;
  }
  
  /*
   * This is the callback that is used to add our wait queue to the
   * target file wakeup lists.
   */
  static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
                                   poll_table *pt)
  {
          struct epitem *epi = ep_item_from_epqueue(pt);
          struct eppoll_entry *pwq;
  
          if (epi->nwait >= 0 && (pwq = kmem_cache_alloc(pwq_cache, GFP_KERNEL))) {
                  init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
                  pwq->whead = whead;
                  pwq->base = epi;
                  add_wait_queue(whead, &pwq->wait);
                  list_add_tail(&pwq->llink, &epi->pwqlist);
                  epi->nwait++;
          } else {
                  /* We have to signal that an error occurred */
                  epi->nwait = -1;
          }
  }
  
  static void ep_rbtree_insert(struct eventpoll *ep, struct epitem *epi)
  {
          int kcmp;
          struct rb_node **p = &ep->rbr.rb_node, *parent = NULL;
          struct epitem *epic;
  
          while (*p) {
                  parent = *p;
                  epic = rb_entry(parent, struct epitem, rbn);
                  kcmp = ep_cmp_ffd(&epi->ffd, &epic->ffd);
                  if (kcmp > 0)
                          p = &parent->rb_right;
                  else
                          p = &parent->rb_left;
          }
          rb_link_node(&epi->rbn, parent, p);
          rb_insert_color(&epi->rbn, &ep->rbr);
  }
  
  
  
  #define PATH_ARR_SIZE 5
  /*
   * These are the number paths of length 1 to 5, that we are allowing to emanate
   * from a single file of interest. For example, we allow 1000 paths of length
   * 1, to emanate from each file of interest. This essentially represents the
   * potential wakeup paths, which need to be limited in order to avoid massive
   * uncontrolled wakeup storms. The common use case should be a single ep which
   * is connected to n file sources. In this case each file source has 1 path
   * of length 1. Thus, the numbers below should be more than sufficient. These
   * path limits are enforced during an EPOLL_CTL_ADD operation, since a modify
   * and delete can't add additional paths. Protected by the epmutex.
   */
  static const int path_limits[PATH_ARR_SIZE] = { 1000, 500, 100, 50, 10 };
  static int path_count[PATH_ARR_SIZE];
  
  static int path_count_inc(int nests)
  {
          /* Allow an arbitrary number of depth 1 paths */
          if (nests == 0)
                  return 0;
  
          if (++path_count[nests] > path_limits[nests])
                  return -1;
          return 0;
  }
  
  static void path_count_init(void)
  {
          int i;
  
          for (i = 0; i < PATH_ARR_SIZE; i++)
                  path_count[i] = 0;
  }
  
  static int reverse_path_check_proc(void *priv, void *cookie, int call_nests)
  {
          int error = 0;
          struct file *file = priv;
          struct file *child_file;
          struct epitem *epi;
  
          list_for_each_entry(epi, &file->f_ep_links, fllink) {
                  child_file = epi->ep->file;
                  if (is_file_epoll(child_file)) {
                          if (list_empty(&child_file->f_ep_links)) {
                                  if (path_count_inc(call_nests)) {
                                          error = -1;
                                          break;
                                  }
                          } else {
                                  error = ep_call_nested(&poll_loop_ncalls,
                                                          EP_MAX_NESTS,
                                                          reverse_path_check_proc,
                                                          child_file, child_file,
                                                          current);
                          }
                          if (error != 0)
                                  break;
                  } else {
                          printk(KERN_ERR "reverse_path_check_proc: "
                                  "file is not an ep!\n");
                  }
          }
          return error;
  }
  
  /**
   * reverse_path_check - The tfile_check_list is list of file *, which have
   *                      links that are proposed to be newly added. We need to
   *                      make sure that those added links don't add too many
   *                      paths such that we will spend all our time waking up
   *                      eventpoll objects.
   *
   * Returns: Returns zero if the proposed links don't create too many paths,
   *          -1 otherwise.
   */
  static int reverse_path_check(void)
  {
          int error = 0;
          struct file *current_file;
  
          /* let's call this for all tfiles */
          list_for_each_entry(current_file, &tfile_check_list, f_tfile_llink) {
                  path_count_init();
                  error = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
                                          reverse_path_check_proc, current_file,
                                          current_file, current);
                  if (error)
                          break;
          }
          return error;
  }
  
  static int ep_create_wakeup_source(struct epitem *epi)
  {
          const char *name;
  
          if (!epi->ep->ws) {
                  epi->ep->ws = wakeup_source_register("eventpoll");
                  if (!epi->ep->ws)
                          return -ENOMEM;
          }
  
          name = epi->ffd.file->f_path.dentry->d_name.name;
          epi->ws = wakeup_source_register(name);
          if (!epi->ws)
                  return -ENOMEM;
  
          return 0;
  }
  
  static void ep_destroy_wakeup_source(struct epitem *epi)
  {
          wakeup_source_unregister(epi->ws);
          epi->ws = NULL;
  }
  
  /*
   * Must be called with "mtx" held.
   */
  static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
                       struct file *tfile, int fd)
  {
          int error, revents, pwake = 0;
          unsigned long flags;
          long user_watches;
          struct epitem *epi;
          struct ep_pqueue epq;
  
          user_watches = atomic_long_read(&ep->user->epoll_watches);
          if (unlikely(user_watches >= max_user_watches))
                  return -ENOSPC;
          if (!(epi = kmem_cache_alloc(epi_cache, GFP_KERNEL)))
                  return -ENOMEM;
  
          /* Item initialization follow here ... */
          INIT_LIST_HEAD(&epi->rdllink);
          INIT_LIST_HEAD(&epi->fllink);
          INIT_LIST_HEAD(&epi->pwqlist);
          epi->ep = ep;
          ep_set_ffd(&epi->ffd, tfile, fd);
          epi->event = *event;
          epi->nwait = 0;
          epi->next = EP_UNACTIVE_PTR;
          if (epi->event.events & EPOLLWAKEUP) {
                  error = ep_create_wakeup_source(epi);
                  if (error)
                          goto error_create_wakeup_source;
          } else {
                  epi->ws = NULL;
          }
  
          /* Initialize the poll table using the queue callback */
          epq.epi = epi;
          init_poll_funcptr(&epq.pt, ep_ptable_queue_proc);
          epq.pt._key = event->events;
  
          /*
           * Attach the item to the poll hooks and get current event bits.
           * We can safely use the file* here because its usage count has
           * been increased by the caller of this function. Note that after
           * this operation completes, the poll callback can start hitting
           * the new item.
           */
          revents = tfile->f_op->poll(tfile, &epq.pt);
  
          /*
           * We have to check if something went wrong during the poll wait queue
           * install process. Namely an allocation for a wait queue failed due
           * high memory pressure.
           */
          error = -ENOMEM;
          if (epi->nwait < 0)
                  goto error_unregister;
  
          /* Add the current item to the list of active epoll hook for this file */
          spin_lock(&tfile->f_lock);
          list_add_tail(&epi->fllink, &tfile->f_ep_links);
          spin_unlock(&tfile->f_lock);
  
          /*
           * Add the current item to the RB tree. All RB tree operations are
           * protected by "mtx", and ep_insert() is called with "mtx" held.
           */
          ep_rbtree_insert(ep, epi);
  
          /* now check if we've created too many backpaths */
          error = -EINVAL;
          if (reverse_path_check())
                  goto error_remove_epi;
  
          /* We have to drop the new item inside our item list to keep track of it */
          spin_lock_irqsave(&ep->lock, flags);
  
          /* If the file is already "ready" we drop it inside the ready list */
          if ((revents & event->events) && !ep_is_linked(&epi->rdllink)) {
                  list_add_tail(&epi->rdllink, &ep->rdllist);
                  __pm_stay_awake(epi->ws);
  
                  /* Notify waiting tasks that events are available */
                  if (waitqueue_active(&ep->wq))
                          wake_up_locked(&ep->wq);
                  if (waitqueue_active(&ep->poll_wait))
                          pwake++;
          }
  
          spin_unlock_irqrestore(&ep->lock, flags);
  
          atomic_long_inc(&ep->user->epoll_watches);
  
          /* We have to call this outside the lock */
          if (pwake)
                  ep_poll_safewake(&ep->poll_wait);
  
          return 0;
  
  error_remove_epi:
          spin_lock(&tfile->f_lock);
          if (ep_is_linked(&epi->fllink))
                  list_del_init(&epi->fllink);
          spin_unlock(&tfile->f_lock);
  
          rb_erase(&epi->rbn, &ep->rbr);
  
  error_unregister:
          ep_unregister_pollwait(ep, epi);
  
          /*
           * We need to do this because an event could have been arrived on some
           * allocated wait queue. Note that we don't care about the ep->ovflist
           * list, since that is used/cleaned only inside a section bound by "mtx".
           * And ep_insert() is called with "mtx" held.
           */
          spin_lock_irqsave(&ep->lock, flags);
          if (ep_is_linked(&epi->rdllink))
                  list_del_init(&epi->rdllink);
          spin_unlock_irqrestore(&ep->lock, flags);
  
          wakeup_source_unregister(epi->ws);
  
  error_create_wakeup_source:
          kmem_cache_free(epi_cache, epi);
  
          return error;
  }
  
  /*
   * Modify the interest event mask by dropping an event if the new mask
   * has a match in the current file status. Must be called with "mtx" held.
   */
  static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_event *event)
  {
          int pwake = 0;
          unsigned int revents;
          poll_table pt;
  
          init_poll_funcptr(&pt, NULL);
  
          /*
           * Set the new event interest mask before calling f_op->poll();
           * otherwise we might miss an event that happens between the
           * f_op->poll() call and the new event set registering.
           */
          epi->event.events = event->events; /* need barrier below */
          pt._key = event->events;
          epi->event.data = event->data; /* protected by mtx */
          if (epi->event.events & EPOLLWAKEUP) {
                  if (!epi->ws)
                          ep_create_wakeup_source(epi);
          } else if (epi->ws) {
                  ep_destroy_wakeup_source(epi);
          }
  
          /*
           * The following barrier has two effects:
           *
           * 1) Flush epi changes above to other CPUs.  This ensures
           *    we do not miss events from ep_poll_callback if an
           *    event occurs immediately after we call f_op->poll().
           *    We need this because we did not take ep->lock while
           *    changing epi above (but ep_poll_callback does take
           *    ep->lock).
           *
           * 2) We also need to ensure we do not miss _past_ events
           *    when calling f_op->poll().  This barrier also
           *    pairs with the barrier in wq_has_sleeper (see
           *    comments for wq_has_sleeper).
           *
           * This barrier will now guarantee ep_poll_callback or f_op->poll
           * (or both) will notice the readiness of an item.
           */
          smp_mb();
  
          /*
           * Get current event bits. We can safely use the file* here because
           * its usage count has been increased by the caller of this function.
           */
          revents = epi->ffd.file->f_op->poll(epi->ffd.file, &pt);
  
          /*
           * If the item is "hot" and it is not registered inside the ready
           * list, push it inside.
           */
          if (revents & event->events) {
                  spin_lock_irq(&ep->lock);
                  if (!ep_is_linked(&epi->rdllink)) {
                          list_add_tail(&epi->rdllink, &ep->rdllist);
                          __pm_stay_awake(epi->ws);
  
                          /* Notify waiting tasks that events are available */
                          if (waitqueue_active(&ep->wq))
                                  wake_up_locked(&ep->wq);
                          if (waitqueue_active(&ep->poll_wait))
                                  pwake++;
                  }
                  spin_unlock_irq(&ep->lock);
          }
  
          /* We have to call this outside the lock */
          if (pwake)
                  ep_poll_safewake(&ep->poll_wait);
  
          return 0;
  }
  
  static int ep_send_events_proc(struct eventpoll *ep, struct list_head *head,
                                 void *priv)
  {
          struct ep_send_events_data *esed = priv;
          int eventcnt;
          unsigned int revents;
          struct epitem *epi;
          struct epoll_event __user *uevent;
          poll_table pt;
  
          init_poll_funcptr(&pt, NULL);
  
          /*
           * We can loop without lock because we are passed a task private list.
           * Items cannot vanish during the loop because ep_scan_ready_list() is
           * holding "mtx" during this call.
           */
          for (eventcnt = 0, uevent = esed->events;
               !list_empty(head) && eventcnt < esed->maxevents;) {
                  epi = list_first_entry(head, struct epitem, rdllink);
  
                  /*
                   * Activate ep->ws before deactivating epi->ws to prevent
                   * triggering auto-suspend here (in case we reactive epi->ws
                   * below).
                   *
                   * This could be rearranged to delay the deactivation of epi->ws
                   * instead, but then epi->ws would temporarily be out of sync
                   * with ep_is_linked().
                   */
                  if (epi->ws && epi->ws->active)
                          __pm_stay_awake(ep->ws);
                  __pm_relax(epi->ws);
                  list_del_init(&epi->rdllink);
  
                  pt._key = epi->event.events;
                  revents = epi->ffd.file->f_op->poll(epi->ffd.file, &pt) &
                          epi->event.events;
  
                  /*
                   * If the event mask intersect the caller-requested one,
                   * deliver the event to userspace. Again, ep_scan_ready_list()
                   * is holding "mtx", so no operations coming from userspace
                   * can change the item.
                   */
                  if (revents) {
                          if (__put_user(revents, &uevent->events) ||
                              __put_user(epi->event.data, &uevent->data)) {
                                  list_add(&epi->rdllink, head);
                                  __pm_stay_awake(epi->ws);
                                  return eventcnt ? eventcnt : -EFAULT;
                          }
                          eventcnt++;
                          uevent++;
                          if (epi->event.events & EPOLLONESHOT)
                                  epi->event.events &= EP_PRIVATE_BITS;
                          else if (!(epi->event.events & EPOLLET)) {
                                  /*
                                   * If this file has been added with Level
                                   * Trigger mode, we need to insert back inside
                                   * the ready list, so that the next call to
                                   * epoll_wait() will check again the events
                                   * availability. At this point, no one can insert
                                   * into ep->rdllist besides us. The epoll_ctl()
                                   * callers are locked out by
                                   * ep_scan_ready_list() holding "mtx" and the
                                   * poll callback will queue them in ep->ovflist.
                                   */
                                  list_add_tail(&epi->rdllink, &ep->rdllist);
                                  __pm_stay_awake(epi->ws);
                          }
                  }
          }
  
          return eventcnt;
  }
  
  static int ep_send_events(struct eventpoll *ep,
                            struct epoll_event __user *events, int maxevents)
  {
          struct ep_send_events_data esed;
  
          esed.maxevents = maxevents;
          esed.events = events;
  
          return ep_scan_ready_list(ep, ep_send_events_proc, &esed, 0);
  }
  
  static inline struct timespec ep_set_mstimeout(long ms)
  {
          struct timespec now, ts = {
                  .tv_sec = ms / MSEC_PER_SEC,
                  .tv_nsec = NSEC_PER_MSEC * (ms % MSEC_PER_SEC),
          };
  
          ktime_get_ts(&now);
          return timespec_add_safe(now, ts);
  }
  
  /**
   * ep_poll - Retrieves ready events, and delivers them to the caller supplied
   *           event buffer.
   *
   * @ep: Pointer to the eventpoll context.
   * @events: Pointer to the userspace buffer where the ready events should be
   *          stored.
   * @maxevents: Size (in terms of number of events) of the caller event buffer.
   * @timeout: Maximum timeout for the ready events fetch operation, in
   *           milliseconds. If the @timeout is zero, the function will not block,
   *           while if the @timeout is less than zero, the function will block
   *           until at least one event has been retrieved (or an error
   *           occurred).
   *
   * Returns: Returns the number of ready events which have been fetched, or an
   *          error code, in case of error.
   */
  static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
                     int maxevents, long timeout)
  {
          int res = 0, eavail, timed_out = 0;
          unsigned long flags;
          long slack = 0;
          wait_queue_t wait;
          ktime_t expires, *to = NULL;
  
          if (timeout > 0) {
                  struct timespec end_time = ep_set_mstimeout(timeout);
  
                  slack = select_estimate_accuracy(&end_time);
                  to = &expires;
                  *to = timespec_to_ktime(end_time);
          } else if (timeout == 0) {
                  /*
                   * Avoid the unnecessary trip to the wait queue loop, if the
                   * caller specified a non blocking operation.
                   */
                  timed_out = 1;
                  spin_lock_irqsave(&ep->lock, flags);
                  goto check_events;
          }
  
  fetch_events:
          spin_lock_irqsave(&ep->lock, flags);
  
          if (!ep_events_available(ep)) {
                  /*
                   * We don't have any available event to return to the caller.
                   * We need to sleep here, and we will be wake up by
                   * ep_poll_callback() when events will become available.
                   */
                  init_waitqueue_entry(&wait, current);
                  __add_wait_queue_exclusive(&ep->wq, &wait);
  
                  for (;;) {
                          /*
                           * We don't want to sleep if the ep_poll_callback() sends us
                           * a wakeup in between. That's why we set the task state
                           * to TASK_INTERRUPTIBLE before doing the checks.
                           */
                          set_current_state(TASK_INTERRUPTIBLE);
                          if (ep_events_available(ep) || timed_out)
                                  break;
                          if (signal_pending(current)) {
                                  res = -EINTR;
                                  break;
                          }
  
                          spin_unlock_irqrestore(&ep->lock, flags);
                          if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
                                  timed_out = 1;
  
                          spin_lock_irqsave(&ep->lock, flags);
                  }
                  __remove_wait_queue(&ep->wq, &wait);
  
                  set_current_state(TASK_RUNNING);
          }
  check_events:
          /* Is it worth to try to dig for events ? */
          eavail = ep_events_available(ep);
  
          spin_unlock_irqrestore(&ep->lock, flags);
  
          /*
           * Try to transfer events to user space. In case we get 0 events and
           * there's still timeout left over, we go trying again in search of
           * more luck.
           */
          if (!res && eavail &&
              !(res = ep_send_events(ep, events, maxevents)) && !timed_out)
                  goto fetch_events;
  
          return res;
  }
  
  /**
   * ep_loop_check_proc - Callback function to be passed to the @ep_call_nested()
   *                      API, to verify that adding an epoll file inside another
   *                      epoll structure, does not violate the constraints, in
   *                      terms of closed loops, or too deep chains (which can
   *                      result in excessive stack usage).
   *
   * @priv: Pointer to the epoll file to be currently checked.
   * @cookie: Original cookie for this call. This is the top-of-the-chain epoll
   *          data structure pointer.
   * @call_nests: Current dept of the @ep_call_nested() call stack.
   *
   * Returns: Returns zero if adding the epoll @file inside current epoll
   *          structure @ep does not violate the constraints, or -1 otherwise.
   */
  static int ep_loop_check_proc(void *priv, void *cookie, int call_nests)
  {
          int error = 0;
          struct file *file = priv;
          struct eventpoll *ep = file->private_data;
          struct eventpoll *ep_tovisit;
          struct rb_node *rbp;
          struct epitem *epi;
  
          mutex_lock_nested(&ep->mtx, call_nests + 1);
          ep->visited = 1;
          list_add(&ep->visited_list_link, &visited_list);
          for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
                  epi = rb_entry(rbp, struct epitem, rbn);
                  if (unlikely(is_file_epoll(epi->ffd.file))) {
                          ep_tovisit = epi->ffd.file->private_data;
                          if (ep_tovisit->visited)
                                  continue;
                          error = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
                                          ep_loop_check_proc, epi->ffd.file,
                                          ep_tovisit, current);
                          if (error != 0)
                                  break;
                  } else {
                          /*
                           * If we've reached a file that is not associated with
                           * an ep, then we need to check if the newly added
                           * links are going to add too many wakeup paths. We do
                           * this by adding it to the tfile_check_list, if it's
                           * not already there, and calling reverse_path_check()
                           * during ep_insert().
                           */
                          if (list_empty(&epi->ffd.file->f_tfile_llink))
                                  list_add(&epi->ffd.file->f_tfile_llink,
                                           &tfile_check_list);
                  }
          }
          mutex_unlock(&ep->mtx);
  
          return error;
  }
  
  /**
   * ep_loop_check - Performs a check to verify that adding an epoll file (@file)
   *                 another epoll file (represented by @ep) does not create
   *                 closed loops or too deep chains.
   *
   * @ep: Pointer to the epoll private data structure.
   * @file: Pointer to the epoll file to be checked.
   *
   * Returns: Returns zero if adding the epoll @file inside current epoll
   *          structure @ep does not violate the constraints, or -1 otherwise.
   */
  static int ep_loop_check(struct eventpoll *ep, struct file *file)
  {
          int ret;
          struct eventpoll *ep_cur, *ep_next;
  
          ret = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
                                ep_loop_check_proc, file, ep, current);
          /* clear visited list */
          list_for_each_entry_safe(ep_cur, ep_next, &visited_list,
                                                          visited_list_link) {
                  ep_cur->visited = 0;
                  list_del(&ep_cur->visited_list_link);
          }
          return ret;
  }
  
  static void clear_tfile_check_list(void)
  {
          struct file *file;
  
          /* first clear the tfile_check_list */
          while (!list_empty(&tfile_check_list)) {
                  file = list_first_entry(&tfile_check_list, struct file,
                                          f_tfile_llink);
                  list_del_init(&file->f_tfile_llink);
          }
          INIT_LIST_HEAD(&tfile_check_list);
  }
  
  /*
   * Open an eventpoll file descriptor.
   */
  SYSCALL_DEFINE1(epoll_create1, int, flags)
  {
          int error, fd;
          struct eventpoll *ep = NULL;
          struct file *file;
  
          /* Check the EPOLL_* constant for consistency.  */
          BUILD_BUG_ON(EPOLL_CLOEXEC != O_CLOEXEC);
  
          if (flags & ~EPOLL_CLOEXEC)
                  return -EINVAL;
          /*
           * Create the internal data structure ("struct eventpoll").
           */
          error = ep_alloc(&ep);
          if (error < 0)
                  return error;
          /*
           * Creates all the items needed to setup an eventpoll file. That is,
           * a file structure and a free file descriptor.
           */
          fd = get_unused_fd_flags(O_RDWR | (flags & O_CLOEXEC));
          if (fd < 0) {
                  error = fd;
                  goto out_free_ep;
          }
          file = anon_inode_getfile("[eventpoll]", &eventpoll_fops, ep,
                                   O_RDWR | (flags & O_CLOEXEC));
          if (IS_ERR(file)) {
                  error = PTR_ERR(file);
                  goto out_free_fd;
          }
          ep->file = file;
          fd_install(fd, file);
          return fd;
  
  out_free_fd:
          put_unused_fd(fd);
  out_free_ep:
          ep_free(ep);
          return error;
  }
  
  SYSCALL_DEFINE1(epoll_create, int, size)
  {
          if (size <= 0)
                  return -EINVAL;
  
          return sys_epoll_create1(0);
  }
  
  /*
   * The following function implements the controller interface for
   * the eventpoll file that enables the insertion/removal/change of
   * file descriptors inside the interest set.
   */
  SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
                  struct epoll_event __user *, event)
  {
          int error;
          int did_lock_epmutex = 0;
          struct file *file, *tfile;
          struct eventpoll *ep;
          struct epitem *epi;
          struct epoll_event epds;
  
          error = -EFAULT;
          if (ep_op_has_event(op) &&
              copy_from_user(&epds, event, sizeof(struct epoll_event)))
                  goto error_return;
  
          /* Get the "struct file *" for the eventpoll file */
          error = -EBADF;
          file = fget(epfd);
          if (!file)
                  goto error_return;
  
          /* Get the "struct file *" for the target file */
          tfile = fget(fd);
          if (!tfile)
                  goto error_fput;
  
          /* The target file descriptor must support poll */
          error = -EPERM;
          if (!tfile->f_op || !tfile->f_op->poll)
                  goto error_tgt_fput;
  
          /* Check if EPOLLWAKEUP is allowed */
          if ((epds.events & EPOLLWAKEUP) && !capable(CAP_BLOCK_SUSPEND))
                  epds.events &= ~EPOLLWAKEUP;
  
          /*
           * We have to check that the file structure underneath the file descriptor
           * the user passed to us _is_ an eventpoll file. And also we do not permit
           * adding an epoll file descriptor inside itself.
           */
          error = -EINVAL;
          if (file == tfile || !is_file_epoll(file))
                  goto error_tgt_fput;
  
          /*
           * At this point it is safe to assume that the "private_data" contains
           * our own data structure.
           */
          ep = file->private_data;
  
          /*
           * When we insert an epoll file descriptor, inside another epoll file
           * descriptor, there is the change of creating closed loops, which are
           * better be handled here, than in more critical paths. While we are
           * checking for loops we also determine the list of files reachable
           * and hang them on the tfile_check_list, so we can check that we
           * haven't created too many possible wakeup paths.
           *
           * We need to hold the epmutex across both ep_insert and ep_remove
           * b/c we want to make sure we are looking at a coherent view of
           * epoll network.
           */
          if (op == EPOLL_CTL_ADD || op == EPOLL_CTL_DEL) {
                  mutex_lock(&epmutex);
                  did_lock_epmutex = 1;
          }
          if (op == EPOLL_CTL_ADD) {
                  if (is_file_epoll(tfile)) {
                          error = -ELOOP;
                          if (ep_loop_check(ep, tfile) != 0) {
                                  clear_tfile_check_list();
                                  goto error_tgt_fput;
                          }
                  } else
                          list_add(&tfile->f_tfile_llink, &tfile_check_list);
          }
  
          mutex_lock_nested(&ep->mtx, 0);
  
          /*
           * Try to lookup the file inside our RB tree, Since we grabbed "mtx"
           * above, we can be sure to be able to use the item looked up by
           * ep_find() till we release the mutex.
           */
          epi = ep_find(ep, tfile, fd);
  
          error = -EINVAL;
          switch (op) {
          case EPOLL_CTL_ADD:
                  if (!epi) {
                          epds.events |= POLLERR | POLLHUP;
                          error = ep_insert(ep, &epds, tfile, fd);
                  } else
                          error = -EEXIST;
                  clear_tfile_check_list();
                  break;
          case EPOLL_CTL_DEL:
                  if (epi)
                          error = ep_remove(ep, epi);
                  else
                          error = -ENOENT;
                  break;
          case EPOLL_CTL_MOD:
                  if (epi) {
                          epds.events |= POLLERR | POLLHUP;
                          error = ep_modify(ep, epi, &epds);
                  } else
                          error = -ENOENT;
                  break;
          }
          mutex_unlock(&ep->mtx);
  
  error_tgt_fput:
          if (did_lock_epmutex)
                  mutex_unlock(&epmutex);
  
          fput(tfile);
  error_fput:
          fput(file);
  error_return:
  
          return error;
  }
  
  /*
   * Implement the event wait interface for the eventpoll file. It is the kernel
   * part of the user space epoll_wait(2).
   */
  SYSCALL_DEFINE4(epoll_wait, int, epfd, struct epoll_event __user *, events,
                  int, maxevents, int, timeout)
  {
          int error;
          struct fd f;
          struct eventpoll *ep;
  
          /* The maximum number of event must be greater than zero */
          if (maxevents <= 0 || maxevents > EP_MAX_EVENTS)
                  return -EINVAL;
  
          /* Verify that the area passed by the user is writeable */
          if (!access_ok(VERIFY_WRITE, events, maxevents * sizeof(struct epoll_event)))
                  return -EFAULT;
  
          /* Get the "struct file *" for the eventpoll file */
          f = fdget(epfd);
          if (!f.file)
                  return -EBADF;
  
          /*
           * We have to check that the file structure underneath the fd
           * the user passed to us _is_ an eventpoll file.
           */
          error = -EINVAL;
          if (!is_file_epoll(f.file))
                  goto error_fput;
  
          /*
           * At this point it is safe to assume that the "private_data" contains
           * our own data structure.
           */
          ep = f.file->private_data;
  
          /* Time to fish for events ... */
          error = ep_poll(ep, events, maxevents, timeout);
  
  error_fput:
          fdput(f);
          return error;
  }
  
  /*
   * Implement the event wait interface for the eventpoll file. It is the kernel
   * part of the user space epoll_pwait(2).
   */
  SYSCALL_DEFINE6(epoll_pwait, int, epfd, struct epoll_event __user *, events,
                  int, maxevents, int, timeout, const sigset_t __user *, sigmask,
                  size_t, sigsetsize)
  {
          int error;
          sigset_t ksigmask, sigsaved;
  
          /*
           * If the caller wants a certain signal mask to be set during the wait,
           * we apply it here.
           */
          if (sigmask) {
                  if (sigsetsize != sizeof(sigset_t))
                          return -EINVAL;
                  if (copy_from_user(&ksigmask, sigmask, sizeof(ksigmask)))
                          return -EFAULT;
                  sigdelsetmask(&ksigmask, sigmask(SIGKILL) | sigmask(SIGSTOP));
                  sigprocmask(SIG_SETMASK, &ksigmask, &sigsaved);
          }
  
          error = sys_epoll_wait(epfd, events, maxevents, timeout);
  
          /*
           * If we changed the signal mask, we need to restore the original one.
           * In case we've got a signal while waiting, we do not restore the
           * signal mask yet, and we allow do_signal() to deliver the signal on
           * the way back to userspace, before the signal mask is restored.
           */
          if (sigmask) {
                  if (error == -EINTR) {
                          memcpy(&current->saved_sigmask, &sigsaved,
                                 sizeof(sigsaved));
                          set_restore_sigmask();
                  } else
                          sigprocmask(SIG_SETMASK, &sigsaved, NULL);
          }
  
          return error;
  }
  
  static int __init eventpoll_init(void)
  {
          struct sysinfo si;
  
          si_meminfo(&si);
          /*
           * Allows top 4% of lomem to be allocated for epoll watches (per user).
           */
          max_user_watches = (((si.totalram - si.totalhigh) / 25) << PAGE_SHIFT) /
                  EP_ITEM_COST;
          BUG_ON(max_user_watches < 0);
  
          /*
           * Initialize the structure used to perform epoll file descriptor
           * inclusion loops checks.
           */
          ep_nested_calls_init(&poll_loop_ncalls);
  
          /* Initialize the structure used to perform safe poll wait head wake ups */
          ep_nested_calls_init(&poll_safewake_ncalls);
  
          /* Initialize the structure used to perform file's f_op->poll() calls */
          ep_nested_calls_init(&poll_readywalk_ncalls);
  
          /* Allocates slab cache used to allocate "struct epitem" items */
          epi_cache = kmem_cache_create("eventpoll_epi", sizeof(struct epitem),
                          0, SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL);
  
          /* Allocates slab cache used to allocate "struct eppoll_entry" */
          pwq_cache = kmem_cache_create("eventpoll_pwq",
                          sizeof(struct eppoll_entry), 0, SLAB_PANIC, NULL);
  
          return 0;
  }
  fs_initcall(eventpoll_init);

Cache object: 3f620c324cd5368edea9f87507f42760