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>

1 files changed, 8 insertions, 2 deletions

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);