Re: [PATCH 1/2] workqueue: rescuer_thread() processes all pwqs before exit

[Tejun Heo <tj@kernel.org>]

Hello,

On Thu, Apr 17, 2014 at 07:34:08AM +0800, Lai Jiangshan wrote:
> Before the rescuer is picked to running, the works of the @pwq
> may be processed by some other workers, and destroy_workqueue()
> may called at the same time. This may result a nasty situation
> that rescuer may exit with non-empty mayday list.
> 
> It is no harm currently, destroy_workqueue() can safely to free
> them all(workqueue&pwqs) togerther, since the rescuer is stopped.
> No rescuer nor mayday-timer can access the mayday list.
> 
> But it is nasty and error-prone in future develop. Fix it.
> 
> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> ---
>  kernel/workqueue.c |   12 ++++++------
>  1 files changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 0ee63af..832125f 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -2409,12 +2409,6 @@ static int rescuer_thread(void *__rescuer)
>  repeat:
>  	set_current_state(TASK_INTERRUPTIBLE);
>  
> -	if (kthread_should_stop()) {
> -		__set_current_state(TASK_RUNNING);
> -		rescuer->task->flags &= ~PF_WQ_WORKER;
> -		return 0;
> -	}
> -
>  	/* see whether any pwq is asking for help */
>  	spin_lock_irq(&wq_mayday_lock);
>  
> @@ -2459,6 +2453,12 @@ repeat:
>  
>  	spin_unlock_irq(&wq_mayday_lock);
>  
> +	if (kthread_should_stop()) {
> +		__set_current_state(TASK_RUNNING);
> +		rescuer->task->flags &= ~PF_WQ_WORKER;
> +		return 0;
> +	}
> +

I don't think this is reliable.  What if mayday requests take place
between wq_mayday_lock and kthread_should_stop() check?  We'll
probably need to run through mayday list after checking should_stop.

Thanks.

-- 
tejun