Re: [PATCH] sched: hoist ASSERT_EXCLUSIVE_WRITER(p->on_rq) above WRITE_ONCE

[Phil Auld <pauld@redhat.com>]

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@nutanix.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.


Reviewed-by: Phil Auld <pauld@redhat.com>


Cheers,
Phil

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