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Date: Thu, 22 Jan 2009 18:47:44 +0100
Message-ID: <c62985530901220947r7cae081bj8660603912769baa@mail.gmail.com>
Subject: Re: [PATCH 2/3] workqueue: not allow recursion run_workqueue
From: =?ISO-8859-1?Q?Fr=E9d=E9ric_Weisbecker?= <fweisbec@gmail.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>, Ingo Molnar <mingo@elte.hu>,
       Andrew Morton <akpm@linux-foundation.org>,
       Peter Zijlstra <peterz@infradead.org>,
       Eric Dumazet <dada1@cosmosbay.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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2009/1/22 Oleg Nesterov <oleg@redhat.com>:
> On 01/22, Frederic Weisbecker wrote:
>>
>> On Thu, Jan 22, 2009 at 05:14:24PM +0800, Lai Jiangshan wrote:
>> >  static int flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)
>> >  {
>> > -   int active;
>> > +   int active = 0;
>> > +   struct wq_barrier barr;
>> >
>> > -   if (cwq->thread == current) {
>> > -           /*
>> > -            * Probably keventd trying to flush its own queue. So simply run
>> > -            * it by hand rather than deadlocking.
>> > -            */
>> > -           run_workqueue(cwq);
>> > -           active = 1;
>> > -   } else {
>> > -           struct wq_barrier barr;
>> > +   BUG_ON(cwq->thread == current);
>>
>> Hi Lai,
>>
>> BUG_ON seems perhaps a bit too much for such case. The system
>> will run in an endless loop because of a mistake that will not have
>> necessarily a fatal end.
>
> Confused. Why do you think the system will run in an endless loop?
> cwq-thread will exit.


Because a BUG_ON panics and then spin for ever. Yeah I shoud have said "panic",
sorry... It was just to tell that a BUG_ON is the end...

>
>> WARN_ON should be enough (plus the warn that lockdep will raise
>> too in this case).
>
> and if cwq-thread proceeds after WARN_ON() it will be "lost" anyway
> because it will sleep forever.

You want to say spin forever?
Why would it? cwq->lock is unlocked at this time.
If we keep the usual path:

if (cwq->thread == current) {
		run_workqueue(cwq);
		active = 1;
	}

it shouldn't hurt.

> Not that I think BUG_ON() is much better, except it is more "loud".

I don't think so IMHO, BUG_ON is for critical issues. Here it is not
critical, the workqueue
will flush but lockdep will warn because of recursion.
That's all.

>
> As for the patch itself, I completely agree with Peter.
>
> Oleg.
>
>