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

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Message-ID: <20140319193407.GA11257@laptop.programming.kicks-ass.net>
Date:	Wed, 19 Mar 2014 20:34:07 +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 03:12:52PM -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.

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