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

[Lai Jiangshan <laijs@cn.fujitsu.com>]

On 06/03/2014 10:28 PM, Peter Zijlstra wrote:
> On Tue, Jun 03, 2014 at 08:45:39PM +0800, Lai Jiangshan wrote:
>>
>> Hi, Peter,
>>
>> I rewrote the analyse. (scheduler_ipi() must be called before stopper-task,
>> so the part for workqueue of the old analyse maybe be wrong.)
> 
> But I don't think there is any guarantee we'll do the wakeup before
> running the stop work.

You are right, but the race window in my old analyse is too narrow to
hit the WARN_ON(). so I rewrote the new analyse showing a much bigger window
which can hit the WARN_ON() in workqueue.c

> 
> Suppose the initial task gets queued, and the thing gets send the
> interrupt, meanwhile we'll do the stopper work wakeup !queueing, the
> set_cpus_allowed_ptr() isn't crossing llc boundaries.
> 
> Now, the remote cpu preempts/schedules before the interrupt hits and
> runs the stop task.
> 
> At which point we'll run __migrate_task() while the task is still queued
> on the wake list.
> 
>> ---
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 268a45e..1a198a5 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);
> 
> So while this will close the window somewhat, I don't think its entirely
> closed.