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[tip:sched/core] sched/wait: Fix add_wait_queue() behavioral change

Commit-ID:  c6b9d9a33029014446bd9ed84c1688f6d3d4eab9
Gitweb:     https://git.kernel.org/tip/c6b9d9a33029014446bd9ed84c1688f6d3d4eab9
Author:     Omar Sandoval <osandov@xxxxxx>
AuthorDate: Tue, 5 Dec 2017 23:15:31 -0800
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 6 Dec 2017 19:30:34 +0100

sched/wait: Fix add_wait_queue() behavioral change

The following cleanup commit:

  50816c48997a ("sched/wait: Standardize internal naming of wait-queue entries")

... unintentionally 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.

Beyond a negative performance impact this change in behavior
theoretically also 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@xxxxxx>
Reviewed-by: Jens Axboe <axboe@xxxxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: kernel-team@xxxxxx
Fixes: ("sched/wait: Standardize internal naming of wait-queue entries")
Link: http://lkml.kernel.org/r/a16c8ccffd39bd08fdaa45a5192294c784b803a7.1512544324.git.osandov@xxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
 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 98feab7..929ecb7 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);