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From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Balbir Singh <bsingharora@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Oleg Nesterov <oleg@redhat.com>
Cc: Nicholas Piggin <nicholas.piggin@gmail.com>,
	Alexey Kardashevskiy <aik@ozlabs.ru>
Subject: Re: [RESEND][v2][PATCH] Fix a race between try_to_wake_up() and a woken up task
Date: Mon, 05 Sep 2016 17:14:19 +1000	[thread overview]
Message-ID: <1473059659.2313.45.camel@kernel.crashing.org> (raw)
In-Reply-To: <e02cce7b-d9ca-1ad0-7a61-ea97c7582b37@gmail.com>

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;
>  

  reply	other threads:[~2016-09-05  7:15 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-08-31 10:08 [v2][PATCH] Fix a race between try_to_wake_up() and a woken up task Balbir Singh
2016-09-05  3:16 ` [RESEND][v2][PATCH] " Balbir Singh
2016-09-05  7:14   ` Benjamin Herrenschmidt [this message]
2016-09-05  7:48     ` Peter Zijlstra
2016-09-05  8:24       ` Mike Galbraith
2016-09-05  8:31         ` Benjamin Herrenschmidt
2016-09-05  8:37       ` Balbir Singh
2016-09-05 11:14   ` [tip:sched/urgent] sched/core: " tip-bot for Balbir Singh

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