Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754441Ab2FKMR2 (ORCPT ); Mon, 11 Jun 2012 08:17:28 -0400 Received: from fieldses.org ([174.143.236.118]:37488 "EHLO fieldses.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754402Ab2FKMR0 (ORCPT ); Mon, 11 Jun 2012 08:17:26 -0400 Date: Mon, 11 Jun 2012 08:17:23 -0400 From: "J. Bruce Fields" To: Peter Zijlstra Cc: Linus Torvalds , Dave Jones , Al Viro , Linux Kernel , Miklos Szeredi , Jan Kara Subject: Re: processes hung after sys_renameat, and 'missing' processes Message-ID: <20120611121723.GD7654@fieldses.org> References: <20120603232820.GQ30000@ZenIV.linux.org.uk> <20120606194233.GA1537@redhat.com> <20120606230040.GA18089@redhat.com> <1339064814.23343.14.camel@twins> <1339140698.23343.26.camel@twins> <20120608144602.GA17251@fieldses.org> <1339168114.2507.44.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1339168114.2507.44.camel@laptop> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1534 Lines: 58 On Fri, Jun 08, 2012 at 05:08:34PM +0200, Peter Zijlstra wrote: > On Fri, 2012-06-08 at 10:46 -0400, J. Bruce Fields wrote: > > > > > Sadly, if you get that annotation wrong you can annotate an actual > > > > > deadlock away. > > > > What's a (contrived as you want) example where that happens? > > spinlock_t lock_array[10]; > > void init_array(void) > { > int i; > > for (i = 0; i < ARRAY_SIZE(lock_array); i++) > spin_lock_init(array + i); > } > > void double_lock(int a, int b) > { > spin_lock(lock_array + a); > spin_lock_nested(lock_array + b, SINGLE_DEPTH_NESTING); > } > > The above places all locks in the array in the same class, it then does > a double lock without order, but tells lockdep the nesting is ok. > > A correct version of the double_lock() function would look like: > > void double_lock(int a, int b) > { > if (b < a) > swap(a, b); > > spin_lock(lock_array + a); > spin_lock_nested(lock_array + b, SINGLE_DEPTH_NESTING); > } > > This orders the locks in array order. Got it, thanks! --b. > > > > > > This the reason you have to be very careful when > > > > > annotating stuff. > > > > Or alternatively--what do I need to check before I call > > mutex_lock_nested? > > That the lock order you tell lockdep is ok, is indeed correct. > > -- 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/