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 <wal...@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
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