linux-kernel - Re: workqueue: WARN at at kernel/workqueue.c:2176

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Message-ID: <20140604093907.GC11096@twins.programming.kicks-ass.net>
Date:	Wed, 4 Jun 2014 11:39:07 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	jjherne@...ux.vnet.ibm.com, Sasha Levin <sasha.levin@...cle.com>,
	Tejun Heo <tj@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: workqueue: WARN at at kernel/workqueue.c:2176

On Wed, Jun 04, 2014 at 04:25:15PM +0800, Lai Jiangshan wrote:
> I think the following code works. (inspirited from the sched_ttwu_pending() in migration_call().)
> 
> if p->on_rq == 0 && p->state == TASK_WAKING in __migrate_task() after this patch,
> it means the cpuallowed is changed before __migrate_task() along with other scheduler
> movements happens between sched_ttwu_pending() and __migrate_task().

Yes, this is better. Can you send a full patch with the below comment in
as well, then I can apply.

Thanks!

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 268a45e..277f3bc 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4530,7 +4530,7 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
>  		goto out;
>  
>  	dest_cpu = cpumask_any_and(cpu_active_mask, new_mask);
> -	if (p->on_rq) {
> +	if (p->on_rq || p->state == TASK_WAKING) {
>  		struct migration_arg arg = { p, dest_cpu };
>  		/* Need help from migration thread: drop lock and wait. */
>  		task_rq_unlock(rq, p, &flags);
> @@ -4656,6 +4656,7 @@ static int migration_cpu_stop(void *data)
>  	 * be on another cpu but it doesn't matter.
>  	 */
>  	local_irq_disable();

	/*
	 * We need to explicitly wake pending tasks before running
	 * __migrate_task() such that we will not miss enforcing
	 * cpus_allowed during wakeups, see set_cpus_allowed_ptr()'s
	 * TASK_WAKING test.
	 */

> +	sched_ttwu_pending();
>  	__migrate_task(arg->task, raw_smp_processor_id(), arg->dest_cpu);
>  	local_irq_enable();
>  	return 0;

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