linux-kernel - Re: [PATCH] swait: add missing barrier to swake

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Date:   Fri, 1 Sep 2017 11:23:22 +0200
From:   Andrea Parri <parri.andrea@...il.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] swait: add missing barrier to swake_up

On Fri, Sep 01, 2017 at 04:14:50PM +1000, Nicholas Piggin wrote:
> swake_up and swake_up_all test the swaitqueue outside the lock,
> but they are missing the barrier that would ensure visibility
> of a previous store that sets the wakeup condition with the
> load that tests the swaitqueue. This could lead to a lost wakeup
> if there is memory reordering. Fix this as prescribed by the
> waitqueue_active comments.
> 
> Signed-off-by: Nicholas Piggin <npiggin@...il.com>
> --
> I noticed this when chasing down that rcu hang bug (which
> turned out to not be anything of the sort). I might be missing
> something here and it's safe somehow, but if so then it should
> have a comment where it diverges from normal waitqueues.
> 
> It looks like there's a few callers which are also testing
> swait_active before swake_up without a barrier which look wrong,
> so I must be missing something but I'm not sure what.

Hi Nicholas. I noticed

  35a2897c2a306cca344ca5c0b43416707018f434
  ("sched/wait: Remove the lockless swait_active() check in swake_up*()")

in tip:locking/core.

  Andrea


> 
> Thanks,
> Nick
> ---
>  kernel/sched/swait.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/kernel/sched/swait.c b/kernel/sched/swait.c
> index 3d5610dcce11..9056278001d9 100644
> --- a/kernel/sched/swait.c
> +++ b/kernel/sched/swait.c
> @@ -33,6 +33,11 @@ void swake_up(struct swait_queue_head *q)
>  {
>  	unsigned long flags;
>  
> +	/*
> +	 * See waitqueue_active() comments for checking waiters outside
> +	 * the lock. Same principle applies here.
> +	 */
> +	smp_mb();
>  	if (!swait_active(q))
>  		return;
>  
> @@ -51,6 +56,11 @@ void swake_up_all(struct swait_queue_head *q)
>  	struct swait_queue *curr;
>  	LIST_HEAD(tmp);
>  
> +	/*
> +	 * See waitqueue_active() comments for checking waiters outside
> +	 * the lock. Same principle applies here.
> +	 */
> +	smp_mb();
>  	if (!swait_active(q))
>  		return;
>  
> -- 
> 2.13.3
>