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Date: Wed, 19 Mar 2014 15:43:39 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>, linux-cifs@vger.kernel.org,
        Steve French <sfrench@samba.org>, Clark Williams <williams@redhat.com>,
        "Luis Claudio R. Goncalves" <lclaudio@uudg.org>,
        Thomas Gleixner <tglx@linutronix.de>, Tejun Heo <tj@kernel.org>,
        uobergfe@redhat.com
Subject: Re: [RFC PATCH] cifs: Fix possible deadlock with cifs and work
 queues
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On Wed, 19 Mar 2014 20:34:07 +0100
Peter Zijlstra <peterz@infradead.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.

-- Steve


> 
> There is the kthread_work stuff for if you want a guaranteed worker
> thread.