linux-kernel - Re: [PATCH] core: workqueue: BUG

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Message-ID: <20100203194350.GA13824@redhat.com>
Date:	Wed, 3 Feb 2010 20:43:50 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Simon Kagstrom <simon.kagstrom@...insight.net>
Cc:	linux-kernel@...r.kernel.org, laijs@...fujitsu.com,
	rusty@...tcorp.com.au, tj@...nel.org, akpm@...ux-foundation.org,
	mingo@...e.hu
Subject: Re: [PATCH] core: workqueue: BUG_ON on workqueue recursion

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.

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