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

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Message-ID: <20140320221913.104b275d@gandalf.local.home>
Date:	Thu, 20 Mar 2014 22:19:13 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-cifs@...r.kernel.org,
	Steve French <sfrench@...ba.org>,
	Peter Zijlstra <peterz@...radead.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 Thu, 20 Mar 2014 19:53:46 -0400
Jeff Layton <jlayton@...hat.com> wrote:

> Wait...why does the work running on CPU1 end up blocked on down_read()?
> Is it really getting stuck on the down_write you mention?
> 

rwsems are fair locks. Readers will not block on a reader lock unless
there's a writer waiting. That's the key. As soon as a writer blocks on
a lock that is held by a reader (or multiple readers), new readers
coming in will also block to let the writer get a chance. Otherwise, it
is a unfair lock and the readers can starve the writer.

But people tend to forget that a waiting writer causes readers to block
on each other, and if the reader locks can deadlock each other, they
will deadlock with a writer waiting.

-- Steve

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