From: Michel Lespinasse Date: Tue, 7 May 2013 13:46:00 +0000 (-0700) Subject: rwsem: do not block readers at head of queue if other readers are active X-Git-Url: http://git.cdn.openwrt.org/?a=commitdiff_plain;h=25c39325968bbcebe6cd2a1991228c9dfb48d655;p=openwrt%2Fstaging%2Fblogic.git 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 Reviewed-by: Peter Hurley Acked-by: Davidlohr Bueso Signed-off-by: Linus Torvalds --- diff --git a/lib/rwsem.c b/lib/rwsem.c index bbe48c04f363..61f91ca75e40 100644 --- a/lib/rwsem.c +++ b/lib/rwsem.c @@ -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);