linux-kernel - Re: [PATCH] sched: hoist ASSERT_EXCLUSIVE_WRITER(p->on_rq) above WRITE

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Message-ID: <0C4B7BAD-04EA-4F60-B6D2-A7B2C14E52B7@nutanix.com>
Date: Thu, 14 Nov 2024 19:01:13 +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: hoist ASSERT_EXCLUSIVE_WRITER(p->on_rq) above
 WRITE_ONCE



> On Nov 14, 2024, at 1:57 PM, Phil Auld <pauld@...hat.com> wrote:
> 
> !-------------------------------------------------------------------|
>  CAUTION: External Email
> 
> |-------------------------------------------------------------------!
> 
> On Thu, Nov 14, 2024 at 09:53:52AM -0700 Jon Kohler wrote:
>> In {activate|deactivate}_task(), hoist ASSERT_EXCLUSIVE_WRITER() to be
>> above WRITE_ONCE(p->on_rq), which matches the ordering listed in the
>> KCSAN documentation, kcsan-checks.h code comments, and the usage
>> pattern we already have in __block_task().
>> 
>> Signed-off-by: Jon Kohler <jon@...anix.com>
>> ---
>> kernel/sched/core.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index a1c353a62c56..80a04c36b495 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2066,16 +2066,16 @@ void activate_task(struct rq *rq, struct task_struct *p, int flags)
>> 
>> enqueue_task(rq, p, flags);
>> 
>> - WRITE_ONCE(p->on_rq, TASK_ON_RQ_QUEUED);
>> ASSERT_EXCLUSIVE_WRITER(p->on_rq);
>> + WRITE_ONCE(p->on_rq, TASK_ON_RQ_QUEUED);
>> }
>> 
>> void deactivate_task(struct rq *rq, struct task_struct *p, int flags)
>> {
>> SCHED_WARN_ON(flags & DEQUEUE_SLEEP);
>> 
>> - WRITE_ONCE(p->on_rq, TASK_ON_RQ_MIGRATING);
>> ASSERT_EXCLUSIVE_WRITER(p->on_rq);
>> + WRITE_ONCE(p->on_rq, TASK_ON_RQ_MIGRATING);
>> 
>> /*
>> * Code explicitly relies on TASK_ON_RQ_MIGRATING begin set *before*
>> -- 
>> 2.43.0
>> 
>> 
> 
> This looks fine to me and it makes sense to have the assert before the
> write.  A quick grep showed that this is by no means a universal pattern
> at the moment.
> 

I’d have to imaging having the assert before must be the right way to
do this, just from a logic control flow perspective. I’m happy to fix ’the
others', or do you think I should let them sit there?

> 
> Reviewed-by: Phil Auld <pauld@...hat.com>
> 
> 
> Cheers,
> Phil
> 
> -- 
>