Commit 25c39325 authored May 07, 2013 by Michel Lespinasse Committed by Linus Torvalds May 07, 2013

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 run 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: Michel Lespinasse <walken@google.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent fe6e674c

lib/rwsem.c

+8 −2

Original line number	Diff line number	Diff line
		@@ -163,8 +163,14 @@ 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);