linux-kernel - Re: [PATCH] sched: add READ_ONCE to task_on_rq

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Message-ID: <6A546A91-A80E-4D4C-9898-5EB792E19635@nutanix.com>
Date: Thu, 14 Nov 2024 18:59:14 +0000
From: Jon Kohler <jon@...anix.com>
To: Phil Auld <pauld@...hat.com>
CC: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot
	<vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
        Mel
 Gorman <mgorman@...e.de>,
        Valentin Schneider <vschneid@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: add READ_ONCE to task_on_rq_queued



> On Nov 14, 2024, at 1:52 PM, Phil Auld <pauld@...hat.com> wrote:
> 
> !-------------------------------------------------------------------|
>  CAUTION: External Email
> 
> |-------------------------------------------------------------------!
> 
> On Thu, Nov 14, 2024 at 09:21:28AM -0700 Jon Kohler wrote:
>> task_on_rq_queued read p->on_rq without READ_ONCE, though p->on_rq is
>> set with WRITE_ONCE in {activate|deactivate}_task and smp_store_release
>> in __block_task, and also read with READ_ONCE in task_on_rq_migrating.
>> 
>> Make all of these accesses pair together by adding READ_ONCE in the
>> task_on_rq_queued.
>> 
>> Signed-off-by: Jon Kohler <jon@...anix.com>
>> ---
>> kernel/sched/sched.h | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index c03b3d7b320e..dbbe5ce0dd96 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -2277,7 +2277,7 @@ static inline int task_on_cpu(struct rq *rq, struct task_struct *p)
>> 
>> static inline int task_on_rq_queued(struct task_struct *p)
>> {
>> - return p->on_rq == TASK_ON_RQ_QUEUED;
>> + return READ_ONCE(p->on_rq) == TASK_ON_RQ_QUEUED;
>> }
>> 
>> static inline int task_on_rq_migrating(struct task_struct *p)
>> -- 
>> 2.43.0
>> 
>> 
> 
> That looks right especially when seen with task_on_rq_migrating()
> right below it, as you said.
> 
> Reviewed-by: Phil Auld <pauld@...hat.com>
> 

Thanks Phil - note, I’ve got a v2 coming shortly as I’ve bungled the
author and codeveloped tag from our internal version of this patch.

I’ll get that out, but it will be the exact same code.

> 
> Cheers,
> Phil
> -- 
>