[PATCH v2 12/13] rwsem: do not block readers at head of queue if other readers are active

[Michel Lespinasse <walken@google.com>]

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: Michel Lespinasse <walken@google.com>

---
 lib/rwsem.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/lib/rwsem.c b/lib/rwsem.c
index 09bf03e7808c..4e4c8893dc00 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -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);
-- 
1.8.1.3