linux-kernel - Re: [RFC PATCH] cifs: Fix possible deadlock with cifs and work queues

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Message-ID: <20140319194729.GB11257@laptop.programming.kicks-ass.net>
Date:	Wed, 19 Mar 2014 20:47:29 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-cifs@...r.kernel.org,
	Steve French <sfrench@...ba.org>,
	Clark Williams <williams@...hat.com>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tejun Heo <tj@...nel.org>, uobergfe@...hat.com
Subject: Re: [RFC PATCH] cifs: Fix possible deadlock with cifs and work queues

On Wed, Mar 19, 2014 at 03:43:39PM -0400, Steven Rostedt wrote:
> On Wed, 19 Mar 2014 20:34:07 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Wed, Mar 19, 2014 at 03:12:52PM -0400, Steven Rostedt wrote:
> > > My question to Tejun is, if we create another workqueue, to add the
> > > rdata->work to, would that prevent the above problem? Or what other
> > > fixes can we do?
> > 
> > The way I understand workqueues is that we cannot guarantee concurrency
> > like this. It tries, but there's no guarantee.
> > 
> > WQ_MAX_ACTIVE seems to be a hard upper limit of concurrent workers. So
> > given 511 other blocked works, the described problem will always happen.
> > 
> > Creating another workqueue doesn't actually create more threads.
> 
> But I noticed this:
> 
>  Before patch:
> 
> # ps aux |grep cifs
> root      3119  0.0  0.0      0     0 ?        S<   14:17   0:00 [cifsiod]
> 
>  After patch:
> 
> # ps aux |grep cifs
> root      1109  0.0  0.0      0     0 ?        S<   15:11   0:00 [cifsiod]
> root      1111  0.0  0.0      0     0 ?        S<   15:11   0:00 [cifsiord]
> 
> It looks to me that it does create new threads.

Ah, I think that's because of the MEM_RECLAIM, not sure if that will
normally participate in running works. Its been a long time since I
looked at any of that.

Lets wait for TJ to wake up.
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