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SubjectRe: [RESEND][v2][PATCH] Fix a race between try_to_wake_up() and a woken up task
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On Mon, 2016-09-05 at 13:16 +1000, Balbir Singh wrote:

 .../...
>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Nicholas Piggin <npiggin@gmail.com>

Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

> Signed-off-by: Balbir Singh <bsingharora@gmail.com>
> ---
>  kernel/sched/core.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2a906f2..582c684 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2016,6 +2016,17 @@ try_to_wake_up(struct task_struct *p, unsigned
> int state, int wake_flags)
>   success = 1; /* we're going to change ->state */
>   cpu = task_cpu(p);
>  
> + /*
> +  * Ensure we see on_rq and p_state consistently
> +  *
> +  * For example in __rwsem_down_write_failed(), we have
> +  *    [S] ->on_rq = 1 [L]
> ->state
> +  *    MB  RMB
> +  *    [S] ->state = TASK_UNINTERRUPTIBLE [L] ->on_rq
> +  * In the absence of the RMB p->on_rq can be observed to be
> 0
> +  * and we end up spinning indefinitely in while (p->on_cpu)
> +  */
> + smp_rmb();
>   if (p->on_rq && ttwu_remote(p, wake_flags))
>   goto stat;
>  

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