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

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Date:	Thu, 04 Feb 2010 11:12:57 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Simon Kagstrom <simon.kagstrom@...insight.net>,
	linux-kernel@...r.kernel.org, laijs@...fujitsu.com,
	rusty@...tcorp.com.au, akpm@...ux-foundation.org, mingo@...e.hu
Subject: Re: [PATCH] core: workqueue: BUG_ON on workqueue recursion

Hello,

On 02/04/2010 04:43 AM, Oleg Nesterov wrote:
> 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?

Yeah, I like this one better too.  Even solely for debugging,
WARN_ON() is better as often users don't have reliable ways to gather
kernel log after a BUG_ON().

Thanks.

-- 
tejun
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