[PATCH v4 4/5] locking/rwsem: Improve reader wakeup code

From: Waiman Long
Date: Tue May 17 2016 - 21:27:10 EST


In __rwsem_do_wake(), the reader wakeup code will assume a writer
has stolen the lock if the active reader/writer count is not 0.
However, this is not as reliable an indicator as the original
"< RWSEM_WAITING_BIAS" check. If another reader is present, the code
will still break out and exit even if the writer is gone. This patch
changes it to check the same "< RWSEM_WAITING_BIAS" condition to
reduce the chance of false positive.

Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx>
Reviewed-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
kernel/locking/rwsem-xadd.c | 11 ++++++++---
1 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 007814f..e3a7e06 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -148,9 +148,14 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
try_reader_grant:
oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
- /* A writer stole the lock. Undo our reader grant. */
- if (rwsem_atomic_update(-adjustment, sem) &
- RWSEM_ACTIVE_MASK)
+ /*
+ * If the count is still less than RWSEM_WAITING_BIAS
+ * after removing the adjustment, it is assumed that
+ * a writer has stolen the lock. We have to undo our
+ * reader grant.
+ */
+ if (rwsem_atomic_update(-adjustment, sem)
+ < RWSEM_WAITING_BIAS)
goto out;
/* Last active locker left. Retry waking readers. */
goto try_reader_grant;
--
1.7.1