From 49e4b2bcf7b812e985e65b6c8a0255b1520a6e7e Mon Sep 17 00:00:00 2001 From: Davidlohr Bueso Date: Fri, 30 Jan 2015 01:14:24 -0800 Subject: locking/rwsem: Document barrier need when waking tasks The need for the smp_mb() in __rwsem_do_wake() should be properly documented. Applies to both xadd and spinlock variants. Signed-off-by: Davidlohr Bueso Signed-off-by: Peter Zijlstra (Intel) Cc: Jason Low Cc: Linus Torvalds Cc: Michel Lespinasse Cc: Paul E. McKenney Cc: Tim Chen Link: http://lkml.kernel.org/r/1422609267-15102-3-git-send-email-dave@stgolabs.net Signed-off-by: Ingo Molnar --- kernel/locking/rwsem-xadd.c | 7 +++++++ 1 file changed, 7 insertions(+) (limited to 'kernel/locking/rwsem-xadd.c') diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 2f7cc4076f50..82aba467564a 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -186,6 +186,13 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) waiter = list_entry(next, struct rwsem_waiter, list); next = waiter->list.next; tsk = waiter->task; + /* + * Make sure we do not wakeup the next reader before + * setting the nil condition to grant the next reader; + * otherwise we could miss the wakeup on the other + * side and end up sleeping again. See the pairing + * in rwsem_down_read_failed(). + */ smp_mb(); waiter->task = NULL; wake_up_process(tsk); -- cgit v1.2.3-59-g8ed1b