Re: sched/wait: Fix add_wait_queue() behavioral change

[Ingo Molnar <mingo@kernel.org>]

* Jens Axboe <axboe@kernel.dk> wrote:

> On 2/10/18 7:14 AM, Tetsuo Handa wrote:
> > Recently, we are seeing I/O hungup reports.
> > 
> > I don't know whether a regression introduced by commit 50816c48997af857
> > ("sched/wait: Standardize internal naming of wait-queue entries") is relevant.
> > But shouldn't we backport commit c6b9d9a330290144 ("sched/wait: Fix
> > add_wait_queue() behavioral change") to 4.13+ kernels anyway?
> 
> Yes, it most certainly should!

Indeed, the bug was introduced in v4.13 and the fix was included in v4.15, but 
it's missing from v4.13 and v4.14 - not sure how I missed that.

The fix (also attached below) applies cleanly to both v4.13 and v4.14:

  Acked-by: Ingo Molnar <mingo@kernel.org>

Thanks,

	Ingo

===================>
>From c6b9d9a33029014446bd9ed84c1688f6d3d4eab9 Mon Sep 17 00:00:00 2001
From: Omar Sandoval <osandov@fb.com>
Date: Tue, 5 Dec 2017 23:15:31 -0800
Subject: [PATCH] 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@fb.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel-team@fb.com
Fixes: ("sched/wait: Standardize internal naming of wait-queue entries")
Link: http://lkml.kernel.org/r/a16c8ccffd39bd08fdaa45a5192294c784b803a7.1512544324.git.osandov@fb.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 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);