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From: Valentin Schneider <valentin.schneider@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>,
	Mel Gorman <mgorman@techsingularity.net>,
	Davidlohr Bueso <dave@stgolabs.net>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched: Fix data-race in wakeup
Date: Tue, 17 Nov 2020 19:32:16 +0000	[thread overview]
Message-ID: <jhjo8jvwzi7.mognet@arm.com> (raw)
In-Reply-To: <20201117161318.GP3121392@hirez.programming.kicks-ass.net>


On 17/11/20 16:13, Peter Zijlstra wrote:
> On Tue, Nov 17, 2020 at 03:37:24PM +0000, Valentin Schneider wrote:
>
>> >> +	/*
>> >> +	 * This field must not be in the scheduler word above due to wakelist
>> >> +	 * queueing no longer being serialized by p->on_cpu. However:
>> >> +	 *
>> >> +	 * p->XXX = X;			ttwu()
>> >> +	 * schedule()			  if (p->on_rq && ..) // false
>> >> +	 *   smp_mb__after_spinlock();	  if (smp_load_acquire(&p->on_cpu) && //true
>> >> +	 *   deactivate_task()		      ttwu_queue_wakelist())
>> >> +	 *     p->on_rq = 0;			p->sched_remote_wakeup = Y;
>> >> +	 *
>> >> +	 * guarantees all stores of 'current' are visible before
>> >> +	 * ->sched_remote_wakeup gets used, so it can be in this word.
>> >> +	 */
>> >
>> > Isn't the control dep between that ttwu() p->on_rq read and
>> > p->sched_remote_wakeup write "sufficient"?
>> 
>> smp_acquire__after_ctrl_dep() that is, since we need
>>   ->on_rq load => 'current' bits load + store
>
> I don't think we need that extra barrier; after all, there will be a
> complete schedule() between waking the task and it actually becoming
> current.

Apologies for the messy train of thought; what I was trying to say is that
we have already the following, which AIUI is sufficient:

	* p->XXX = X;			ttwu()
	* schedule()			  if (p->on_rq && ..) // false
	*   smp_mb__after_spinlock();	  smp_acquire__after_ctrl_dep();
	*   deactivate_task()		  ttwu_queue_wakelist()
	*     p->on_rq = 0;		    p->sched_remote_wakeup = Y;


WARNING: multiple messages have this Message-ID (diff)
From: Valentin Schneider <valentin.schneider@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Will Deacon <will@kernel.org>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] sched: Fix data-race in wakeup
Date: Tue, 17 Nov 2020 19:32:16 +0000	[thread overview]
Message-ID: <jhjo8jvwzi7.mognet@arm.com> (raw)
In-Reply-To: <20201117161318.GP3121392@hirez.programming.kicks-ass.net>


On 17/11/20 16:13, Peter Zijlstra wrote:
> On Tue, Nov 17, 2020 at 03:37:24PM +0000, Valentin Schneider wrote:
>
>> >> +	/*
>> >> +	 * This field must not be in the scheduler word above due to wakelist
>> >> +	 * queueing no longer being serialized by p->on_cpu. However:
>> >> +	 *
>> >> +	 * p->XXX = X;			ttwu()
>> >> +	 * schedule()			  if (p->on_rq && ..) // false
>> >> +	 *   smp_mb__after_spinlock();	  if (smp_load_acquire(&p->on_cpu) && //true
>> >> +	 *   deactivate_task()		      ttwu_queue_wakelist())
>> >> +	 *     p->on_rq = 0;			p->sched_remote_wakeup = Y;
>> >> +	 *
>> >> +	 * guarantees all stores of 'current' are visible before
>> >> +	 * ->sched_remote_wakeup gets used, so it can be in this word.
>> >> +	 */
>> >
>> > Isn't the control dep between that ttwu() p->on_rq read and
>> > p->sched_remote_wakeup write "sufficient"?
>> 
>> smp_acquire__after_ctrl_dep() that is, since we need
>>   ->on_rq load => 'current' bits load + store
>
> I don't think we need that extra barrier; after all, there will be a
> complete schedule() between waking the task and it actually becoming
> current.

Apologies for the messy train of thought; what I was trying to say is that
we have already the following, which AIUI is sufficient:

	* p->XXX = X;			ttwu()
	* schedule()			  if (p->on_rq && ..) // false
	*   smp_mb__after_spinlock();	  smp_acquire__after_ctrl_dep();
	*   deactivate_task()		  ttwu_queue_wakelist()
	*     p->on_rq = 0;		    p->sched_remote_wakeup = Y;


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  reply	other threads:[~2020-11-17 19:32 UTC|newest]

Thread overview: 70+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-16  9:10 Loadavg accounting error on arm64 Mel Gorman
2020-11-16  9:10 ` Mel Gorman
2020-11-16 11:49 ` Mel Gorman
2020-11-16 11:49   ` Mel Gorman
2020-11-16 12:00   ` Mel Gorman
2020-11-16 12:00     ` Mel Gorman
2020-11-16 12:53   ` Peter Zijlstra
2020-11-16 12:53     ` Peter Zijlstra
2020-11-16 12:58     ` Peter Zijlstra
2020-11-16 12:58       ` Peter Zijlstra
2020-11-16 15:29       ` Mel Gorman
2020-11-16 15:29         ` Mel Gorman
2020-11-16 16:42         ` Mel Gorman
2020-11-16 16:42           ` Mel Gorman
2020-11-16 16:49         ` Peter Zijlstra
2020-11-16 16:49           ` Peter Zijlstra
2020-11-16 17:24           ` Mel Gorman
2020-11-16 17:24             ` Mel Gorman
2020-11-16 17:41             ` Will Deacon
2020-11-16 17:41               ` Will Deacon
2020-11-16 12:46 ` Peter Zijlstra
2020-11-16 12:46   ` Peter Zijlstra
2020-11-16 12:58   ` Mel Gorman
2020-11-16 12:58     ` Mel Gorman
2020-11-16 13:11 ` Will Deacon
2020-11-16 13:11   ` Will Deacon
2020-11-16 13:37   ` Mel Gorman
2020-11-16 13:37     ` Mel Gorman
2020-11-16 14:20     ` Peter Zijlstra
2020-11-16 14:20       ` Peter Zijlstra
2020-11-16 15:52       ` Mel Gorman
2020-11-16 15:52         ` Mel Gorman
2020-11-16 16:54         ` Peter Zijlstra
2020-11-16 16:54           ` Peter Zijlstra
2020-11-16 17:16           ` Mel Gorman
2020-11-16 17:16             ` Mel Gorman
2020-11-16 19:31       ` Mel Gorman
2020-11-16 19:31         ` Mel Gorman
2020-11-17  8:30         ` [PATCH] sched: Fix data-race in wakeup Peter Zijlstra
2020-11-17  8:30           ` Peter Zijlstra
2020-11-17  9:15           ` Will Deacon
2020-11-17  9:15             ` Will Deacon
2020-11-17  9:29             ` Peter Zijlstra
2020-11-17  9:29               ` Peter Zijlstra
2020-11-17  9:46               ` Peter Zijlstra
2020-11-17  9:46                 ` Peter Zijlstra
2020-11-17 10:36                 ` Will Deacon
2020-11-17 10:36                   ` Will Deacon
2020-11-17 12:52                 ` Valentin Schneider
2020-11-17 12:52                   ` Valentin Schneider
2020-11-17 15:37                   ` Valentin Schneider
2020-11-17 15:37                     ` Valentin Schneider
2020-11-17 16:13                     ` Peter Zijlstra
2020-11-17 16:13                       ` Peter Zijlstra
2020-11-17 19:32                       ` Valentin Schneider [this message]
2020-11-17 19:32                         ` Valentin Schneider
2020-11-18  8:05                         ` Peter Zijlstra
2020-11-18  8:05                           ` Peter Zijlstra
2020-11-18  9:51                           ` Valentin Schneider
2020-11-18  9:51                             ` Valentin Schneider
2020-11-18 13:33               ` Marco Elver
2020-11-18 13:33                 ` Marco Elver
2020-11-17  9:38           ` [PATCH] sched: Fix rq->nr_iowait ordering Peter Zijlstra
2020-11-17  9:38             ` Peter Zijlstra
2020-11-17 11:43             ` Mel Gorman
2020-11-17 11:43               ` Mel Gorman
2020-11-19  9:55             ` [tip: sched/urgent] " tip-bot2 for Peter Zijlstra
2020-11-17 12:40           ` [PATCH] sched: Fix data-race in wakeup Mel Gorman
2020-11-17 12:40             ` Mel Gorman
2020-11-19  9:55           ` [tip: sched/urgent] " tip-bot2 for Peter Zijlstra

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