Re: [PATCH] core: workqueue: BUG_ON on workqueue recursion

[Oleg Nesterov <oleg@redhat.com>]

On 02/03, Simon Kagstrom wrote:
>
> When the workqueue is flushed from workqueue context (recursively), the
> system enters a strange state where things at random (dependent on the
> global workqueue) start misbehaving. For example, for us the console and
> logins locks up while the web server continues running.
>
> Since the system becomes unstable, change this to a BUG_ON instead.

I agree with this patch. We are going to deadlock anyway, if the
condition is true the caller is cwq->current_work, this means
flush_cpu_workqueue() will insert the barrier and hang.

However,

> @@ -482,7 +482,7 @@ static int flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)
>  	int active = 0;
>  	struct wq_barrier barr;
>
> -	WARN_ON(cwq->thread == current);
> +	BUG_ON(cwq->thread == current);

Another option is change the code to do

	if (WARN_ON(cwq->thread == current))
		return;

This gives the kernel chance to survive after the warning.

What do you think?

Oleg.