linux-kernel - Re: [PATCH] sched: Avoid that __wait_on_bit

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Date:	Thu, 4 Aug 2016 16:09:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Bart Van Assche <bart.vanassche@...disk.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	"mingo@...nel.org" <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Neil Brown <neilb@...e.de>,
	Michael Shaver <jmshaver@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Avoid that __wait_on_bit_lock() hangs

On Wed, Aug 03, 2016 at 02:51:23PM -0700, Bart Van Assche wrote:
> So I started testing the patch below that should fix the same hang but
> without triggering any wait list corruption.
> 
> diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
> index f15d6b6..4e3f651 100644
> --- a/kernel/sched/wait.c
> +++ b/kernel/sched/wait.c
> @@ -282,7 +282,7 @@ void abort_exclusive_wait(wait_queue_head_t *q,
> wait_queue_t *wait,
>  	spin_lock_irqsave(&q->lock, flags);
>  	if (!list_empty(&wait->task_list))
>  		list_del_init(&wait->task_list);
> -	else if (waitqueue_active(q))
> +	if (waitqueue_active(q))
>  		__wake_up_locked_key(q, mode, key);
>  	spin_unlock_irqrestore(&q->lock, flags);
>  }

So the problem with this patch is that it will violate the nr_exclusive
semantics in that it can result in too many wakeups -- which is a much
less severe (typically harmless) issue.

We now always wake up the next waiter, even if there wasn't an actual
wakeup we raced against. And if we then also get a wakeup, we can end up
with 2 woken tasks (instead of the nr_exclusive=1).

Now, since wait loops must all deal with spurious wakeups, this ends up
as harmless overhead.

But I'd still like to understand where we loose the wakeup. What are you
doing to reproduce this issue?