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 <[email protected]> Reviewed-by: Peter Hurley <[email protected]> --- 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
