[PATCH] sched/wait: fix add_wait_queue() behavior change

From: Omar Sandoval <osandov@osandov.com>
To: linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>
Cc: kernel-team@fb.com
Subject: [PATCH] sched/wait: fix add_wait_queue() behavior change
Date: Tue,  5 Dec 2017 23:15:31 -0800	[thread overview]
Message-ID: <a16c8ccffd39bd08fdaa45a5192294c784b803a7.1512544324.git.osandov@fb.com> (raw)
In-Reply-To: <CA+55aFwhq-ZyDW2SYhYDg1ytKybwHywiH9jW3UmMu=YAgk694Q@mail.gmail.com>

From: Omar Sandoval <osandov@fb.com>

Commit 50816c48997a ("sched/wait: Standardize internal naming of
wait-queue entries") changed the behavior of add_wait_queue() from
inserting the wait entry at the head of the wait queue to the tail of
the wait queue. That commit was a cleanup and didn't mention any
functional changes so it was likely unintentional. This change in
behavior theoretically breaks wait queues which mix exclusive and
non-exclusive waiters, as non-exclusive waiters will not be woken up if
they are queued behind enough exclusive waiters.

Signed-off-by: Omar Sandoval <osandov@fb.com>
---
Since Ingo hasn't replied, here's a proper patch.

 kernel/sched/wait.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
index 98feab7933c7..929ecb7d6b78 100644
--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -27,7 +27,7 @@ void add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq
 
 	wq_entry->flags &= ~WQ_FLAG_EXCLUSIVE;
 	spin_lock_irqsave(&wq_head->lock, flags);
-	__add_wait_queue_entry_tail(wq_head, wq_entry);
+	__add_wait_queue(wq_head, wq_entry);
 	spin_unlock_irqrestore(&wq_head->lock, flags);
 }
 EXPORT_SYMBOL(add_wait_queue);
-- 
2.15.1
