Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757683Ab2EYNji (ORCPT ); Fri, 25 May 2012 09:39:38 -0400 Received: from merlin.infradead.org ([205.233.59.134]:41112 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755536Ab2EYNjg (ORCPT ); Fri, 25 May 2012 09:39:36 -0400 Subject: Re: [PATCH] tty: tty_mutex: fix lockdep warning in tty_lock_pair(v1) From: Peter Zijlstra To: Ming Lei Cc: Greg Kroah-Hartman , linux-kernel@vger.kernel.org, Alan Cox , Arnd Bergmann In-Reply-To: <1337952533.9783.195.camel@laptop> References: <1337651931-5301-1-git-send-email-ming.lei@canonical.com> <1337952533.9783.195.camel@laptop> Content-Type: text/plain; charset="UTF-8" Date: Fri, 25 May 2012 15:39:21 +0200 Message-ID: <1337953161.9783.201.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.32.2 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1875 Lines: 56 On Fri, 2012-05-25 at 15:28 +0200, Peter Zijlstra wrote: > On Wed, 2012-05-23 at 14:01 +0800, Ming Lei wrote: > > > Even though the patch is applied, there is still one related problem about > > mixing tty_lock_pair with tty_unlock and tty_lock. If tty locks are > > held by calling > > tty_lock_pair, then deadlock warning between legacy_mutex/1 and legacy_mutex > > may be triggered if tty_unlock(tty) and tty_lock(tty) are called later > > when tty < tty2, > > see tty_ldisc_release() in tty_release(). > > This just gives me a head-ache instead of explaining anything. > > Having looked at the source I still don't see how it could possibly > work,.. So the problem with tty_release() -> tty_ldisc_release() is that > tty_ldisc_release() does an unlock/lock of tty. > > However your tty_lock_pair() can still result in tty being subclass 1, > see your else branch, nested case. > > That said, how is this not a real deadlock? If you rely on tty pointer > ordering to avoid deadlocks, you always need to lock them in the same > order. The unlock+lock in ldisc_release violates that. > > If we don't rely on the order, then why bother with the _pair() > primitive? A git grep reveals tty_release() is the only user of tty_lock_pair() and while we hold tty_mutex over the tty_lock_pair() its not held over ldisc_release(). Thus afaict we can create the following deadlock: cpu-A cpu-B lock tty_mutex lock tty lock o_tty unlock tty_mutex unlock tty lock tty_mutex lock tty lock o_tty -> block on A lock tty -> block on B Also, what is that plain call to schedule() doing in tty_release()?! -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/