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Commit d6ccfacb authored by Michel Lespinasse's avatar Michel Lespinasse Committed by Samuel Pascua
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rwsem: do not block readers at head of queue if other readers are active 



This change fixes a race condition where a reader might determine it
needs to block, but by the time it acquires the wait_lock the rwsem
has active readers and no queued waiters.

In this situation the reader can just in parallel with the existing active
readers; it does not need to block until the active readers complete.

Thanks to Peter Hurley for noticing this possible race.

Signed-off-by: default avatarMichel Lespinasse <walken@google.com>
parent 9e3f6b5e

lib/rwsem.c

+8 −2
Original line number Diff line number Diff line
@@ -162,8 +162,14 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) 
	/* 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). */

	if (!(count & RWSEM_ACTIVE_MASK))

	/* If there are no active locks, wake the front queued process(es).

	 *

	 * If there are no writers and we are first in the queue,

	 * wake our own waiter to join the existing active readers !

	 */

	if (count == RWSEM_WAITING_BIAS ||

	    (count > RWSEM_WAITING_BIAS &&

	     adjustment != -RWSEM_ACTIVE_READ_BIAS))

		sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);



	raw_spin_unlock_irq(&sem->wait_lock);