linux-kernel - Re: [RFC][PATCH 1/4] sched: Fix a race between __kthread_bind() and sched

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Date:	Fri, 7 Aug 2015 11:16:08 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, riel@...hat.com, dedekind1@...il.com,
	linux-kernel@...r.kernel.org, mgorman@...e.de, rostedt@...dmis.org,
	juri.lelli@....com, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [RFC][PATCH 1/4] sched: Fix a race between __kthread_bind() and
 sched_setaffinity()

On Fri, Aug 07, 2015 at 04:27:08PM +0200, Peter Zijlstra wrote:
> Which is the rescue thread attaching itself to a pool that needs help,
> and obviously the rescue thread isn't new so kthread_bind doesn't work
> right.
> 
> The best I could come up with is something like the below on top; does
> that work for you? I'll go give it some runtime.
> 
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -1622,11 +1622,15 @@ static struct worker *alloc_worker(int n
>   * cpu-[un]hotplugs.
>   */
>  static void worker_attach_to_pool(struct worker *worker,
> -				   struct worker_pool *pool)
> +				   struct worker_pool *pool,
> +				   bool new)
>  {
>  	mutex_lock(&pool->attach_mutex);
>  
> -	kthread_bind_mask(worker->task, pool->attrs->cpumask);
> +	if (new)
> +		kthread_bind_mask(worker->task, pool->attrs->cpumask);
> +	else
> +		set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
>  
>  	/*
>  	 * The pool->attach_mutex ensures %POOL_DISASSOCIATED remains
> @@ -1712,7 +1716,7 @@ static struct worker *create_worker(stru
>  	set_user_nice(worker->task, pool->attrs->nice);
>  
>  	/* successful, attach the worker to the pool */
> -	worker_attach_to_pool(worker, pool);
> +	worker_attach_to_pool(worker, pool, true);
>  
>  	/* start the newly created worker */
>  	spin_lock_irq(&pool->lock);
> @@ -2241,7 +2245,7 @@ static int rescuer_thread(void *__rescue
>  
>  		spin_unlock_irq(&wq_mayday_lock);
>  
> -		worker_attach_to_pool(rescuer, pool);
> +		worker_attach_to_pool(rescuer, pool, false);

Hmmm... the race condition didn't exist for workqueue in the first
place, right?  As long as the flag is set before the affinity is
configured, there's no race condition.  I think the code was better
before.  Can't we just revert workqueue.c part?

Thanks.

-- 
tejun
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