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authorMichel Lespinasse <walken@google.com>2013-05-07 06:46:00 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2013-05-07 07:20:17 -0700
commit25c39325968bbcebe6cd2a1991228c9dfb48d655 (patch)
tree627e32434338adcf4f7583219e9b4014b738dcc3
parentfe6e674c6187d4f452a679ced7e95262bd517936 (diff)
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>
-rw-r--r--lib/rwsem.c10
1 files changed, 8 insertions, 2 deletions
diff --git a/lib/rwsem.c b/lib/rwsem.c
index bbe48c0..61f91ca 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);