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SubjectRe: [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@gmail.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
>

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