Re: [PATCH 2/3] workqueue: not allow recursion run_workqueue

From: Oleg Nesterov <oleg@redhat.com>
To: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: "Peter Zijlstra" <peterz@infradead.org>,
	"Frédéric Weisbecker" <fweisbec@gmail.com>,
	"Ingo Molnar" <mingo@elte.hu>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Eric Dumazet" <dada1@cosmosbay.com>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/3] workqueue: not allow recursion run_workqueue
Date: Thu, 5 Feb 2009 18:01:56 +0100	[thread overview]
Message-ID: <20090205170156.GA25517@redhat.com> (raw)
In-Reply-To: <498AA0F1.2030003@cn.fujitsu.com>

On 02/05, Lai Jiangshan wrote:
>
> DEADLOCK EXAMPLE for explain my above option:
>
> (work_func0() and work_func1() are work callback, and they
> calls flush_workqueue())
>
> CPU#0					CPU#1
> run_workqueue()                         run_workqueue()
>   work_func0()                            work_func1()
>     flush_workqueue()                       flush_workqueue()
>       flush_cpu_workqueue(0)                  .
>       flush_cpu_workqueue(cpu#1)              flush_cpu_workqueue(cpu#0)
>         waiting work_func1() in cpu#1           waiting work_func0 in cpu#0
>
> DEADLOCK!

I am not sure. Note that when work_func0() calls run_workqueue(),
it will clear cwq->current_work, so another flush_ on CPU#1 will
not wait for work_func0, no?

But anyway. Nobody argues, "if (cwq->thread == current) {...}" code in
flush_cpu_workqueue() is bad and should die. Otrherwise, we should
fix the lockdep warning ;)

The only problem: if we still have the users of this hack, they will
deadlock. But perhaps it is time to fix them.

And, if it was not clear, I do agree with this change. And Peter
seems to agree as well.

Oleg.