FreeBSD/Linux Kernel Cross Reference
sys/lib/rwsem.c

     /* rwsem.c: R/W semaphores: contention handling functions
      *
      * Written by David Howells (dhowells@redhat.com).
      * Derived from arch/i386/kernel/semaphore.c
      */
     #include <linux/rwsem.h>
     #include <linux/sched.h>
     #include <linux/init.h>
     #include <linux/export.h>
    
    /*
     * Initialize an rwsem:
     */
    void __init_rwsem(struct rw_semaphore *sem, const char *name,
                      struct lock_class_key *key)
    {
    #ifdef CONFIG_DEBUG_LOCK_ALLOC
            /*
             * Make sure we are not reinitializing a held semaphore:
             */
            debug_check_no_locks_freed((void *)sem, sizeof(*sem));
            lockdep_init_map(&sem->dep_map, name, key, 0);
    #endif
            sem->count = RWSEM_UNLOCKED_VALUE;
            raw_spin_lock_init(&sem->wait_lock);
            INIT_LIST_HEAD(&sem->wait_list);
    }
    
    EXPORT_SYMBOL(__init_rwsem);
    
    struct rwsem_waiter {
            struct list_head list;
            struct task_struct *task;
            unsigned int flags;
    #define RWSEM_WAITING_FOR_READ  0x00000001
    #define RWSEM_WAITING_FOR_WRITE 0x00000002
    };
    
    /* Wake types for __rwsem_do_wake().  Note that RWSEM_WAKE_NO_ACTIVE and
     * RWSEM_WAKE_READ_OWNED imply that the spinlock must have been kept held
     * since the rwsem value was observed.
     */
    #define RWSEM_WAKE_ANY        0 /* Wake whatever's at head of wait list */
    #define RWSEM_WAKE_NO_ACTIVE  1 /* rwsem was observed with no active thread */
    #define RWSEM_WAKE_READ_OWNED 2 /* rwsem was observed to be read owned */
    
    /*
     * handle the lock release when processes blocked on it that can now run
     * - if we come here from up_xxxx(), then:
     *   - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
     *   - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
     * - there must be someone on the queue
     * - the spinlock must be held by the caller
     * - woken process blocks are discarded from the list after having task zeroed
     * - writers are only woken if downgrading is false
     */
    static struct rw_semaphore *
    __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
    {
            struct rwsem_waiter *waiter;
            struct task_struct *tsk;
            struct list_head *next;
            signed long oldcount, woken, loop, adjustment;
    
            waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
            if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
                    goto readers_only;
    
            if (wake_type == RWSEM_WAKE_READ_OWNED)
                    /* Another active reader was observed, so wakeup is not
                     * likely to succeed. Save the atomic op.
                     */
                    goto out;
    
            /* There's a writer at the front of the queue - try to grant it the
             * write lock.  However, we only wake this writer if we can transition
             * the active part of the count from 0 -> 1
             */
            adjustment = RWSEM_ACTIVE_WRITE_BIAS;
            if (waiter->list.next == &sem->wait_list)
                    adjustment -= RWSEM_WAITING_BIAS;
    
     try_again_write:
            oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
            if (oldcount & RWSEM_ACTIVE_MASK)
                    /* Someone grabbed the sem already */
                    goto undo_write;
    
            /* We must be careful not to touch 'waiter' after we set ->task = NULL.
             * It is an allocated on the waiter's stack and may become invalid at
             * any time after that point (due to a wakeup from another source).
             */
            list_del(&waiter->list);
            tsk = waiter->task;
            smp_mb();
            waiter->task = NULL;
            wake_up_process(tsk);
            put_task_struct(tsk);
            goto out;
   
    readers_only:
           /* If we come here from up_xxxx(), another thread might have reached
            * rwsem_down_failed_common() before we acquired the spinlock and
            * woken up a waiter, making it now active.  We prefer to check for
            * this first in order to not spend too much time with the spinlock
            * held if we're not going to be able to wake up readers in the end.
            *
            * Note that we do not need to update the rwsem count: any writer
            * trying to acquire rwsem will run rwsem_down_write_failed() due
            * to the waiting threads and block trying to acquire the spinlock.
            *
            * We use a dummy atomic update in order to acquire the cache line
            * exclusively since we expect to succeed and run the final rwsem
            * count adjustment pretty soon.
            */
           if (wake_type == RWSEM_WAKE_ANY &&
               rwsem_atomic_update(0, sem) < RWSEM_WAITING_BIAS)
                   /* Someone grabbed the sem for write already */
                   goto out;
   
           /* Grant an infinite number of read locks to the readers at the front
            * of the queue.  Note we increment the 'active part' of the count by
            * the number of readers before waking any processes up.
            */
           woken = 0;
           do {
                   woken++;
   
                   if (waiter->list.next == &sem->wait_list)
                           break;
   
                   waiter = list_entry(waiter->list.next,
                                           struct rwsem_waiter, list);
   
           } while (waiter->flags & RWSEM_WAITING_FOR_READ);
   
           adjustment = woken * RWSEM_ACTIVE_READ_BIAS;
           if (waiter->flags & RWSEM_WAITING_FOR_READ)
                   /* hit end of list above */
                   adjustment -= RWSEM_WAITING_BIAS;
   
           rwsem_atomic_add(adjustment, sem);
   
           next = sem->wait_list.next;
           for (loop = woken; loop > 0; loop--) {
                   waiter = list_entry(next, struct rwsem_waiter, list);
                   next = waiter->list.next;
                   tsk = waiter->task;
                   smp_mb();
                   waiter->task = NULL;
                   wake_up_process(tsk);
                   put_task_struct(tsk);
           }
   
           sem->wait_list.next = next;
           next->prev = &sem->wait_list;
   
    out:
           return sem;
   
           /* undo the change to the active count, but check for a transition
            * 1->0 */
    undo_write:
           if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK)
                   goto out;
           goto try_again_write;
   }
   
   /*
    * wait for a lock to be granted
    */
   static struct rw_semaphore __sched *
   rwsem_down_failed_common(struct rw_semaphore *sem,
                            unsigned int flags, signed long adjustment)
   {
           struct rwsem_waiter waiter;
           struct task_struct *tsk = current;
           signed long count;
   
           set_task_state(tsk, TASK_UNINTERRUPTIBLE);
   
           /* set up my own style of waitqueue */
           raw_spin_lock_irq(&sem->wait_lock);
           waiter.task = tsk;
           waiter.flags = flags;
           get_task_struct(tsk);
   
           if (list_empty(&sem->wait_list))
                   adjustment += RWSEM_WAITING_BIAS;
           list_add_tail(&waiter.list, &sem->wait_list);
   
           /* we're now waiting on the lock, but no longer actively locking */
           count = rwsem_atomic_update(adjustment, sem);
   
           /* If there are no active locks, wake the front queued process(es) up.
            *
            * Alternatively, if we're called from a failed down_write(), there
            * were already threads queued before us and there are no active
            * writers, the lock must be read owned; so we try to wake any read
            * locks that were queued ahead of us. */
           if (count == RWSEM_WAITING_BIAS)
                   sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE);
           else if (count > RWSEM_WAITING_BIAS &&
                    adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
                   sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
   
           raw_spin_unlock_irq(&sem->wait_lock);
   
           /* wait to be given the lock */
           for (;;) {
                   if (!waiter.task)
                           break;
                   schedule();
                   set_task_state(tsk, TASK_UNINTERRUPTIBLE);
           }
   
           tsk->state = TASK_RUNNING;
   
           return sem;
   }
   
   /*
    * wait for the read lock to be granted
    */
   struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
   {
           return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_READ,
                                           -RWSEM_ACTIVE_READ_BIAS);
   }
   
   /*
    * wait for the write lock to be granted
    */
   struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
   {
           return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_WRITE,
                                           -RWSEM_ACTIVE_WRITE_BIAS);
   }
   
   /*
    * handle waking up a waiter on the semaphore
    * - up_read/up_write has decremented the active part of count if we come here
    */
   struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
   {
           unsigned long flags;
   
           raw_spin_lock_irqsave(&sem->wait_lock, flags);
   
           /* do nothing if list empty */
           if (!list_empty(&sem->wait_list))
                   sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
   
           raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
   
           return sem;
   }
   
   /*
    * downgrade a write lock into a read lock
    * - caller incremented waiting part of count and discovered it still negative
    * - just wake up any readers at the front of the queue
    */
   struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
   {
           unsigned long flags;
   
           raw_spin_lock_irqsave(&sem->wait_lock, flags);
   
           /* do nothing if list empty */
           if (!list_empty(&sem->wait_list))
                   sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
   
           raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
   
           return sem;
   }
   
   EXPORT_SYMBOL(rwsem_down_read_failed);
   EXPORT_SYMBOL(rwsem_down_write_failed);
   EXPORT_SYMBOL(rwsem_wake);
   EXPORT_SYMBOL(rwsem_downgrade_wake);

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