Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755513AbbBPQGW (ORCPT ); Mon, 16 Feb 2015 11:06:22 -0500 Received: from cantor2.suse.de ([195.135.220.15]:35073 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753096AbbBPQGU (ORCPT ); Mon, 16 Feb 2015 11:06:20 -0500 Date: Mon, 16 Feb 2015 17:06:15 +0100 (CET) From: Miroslav Benes To: Josh Poimboeuf cc: Jiri Slaby , Seth Jennings , Jiri Kosina , Vojtech Pavlik , Masami Hiramatsu , live-patching@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 8/9] livepatch: allow patch modules to be removed In-Reply-To: <20150213204924.GH27180@treble.redhat.com> Message-ID: References: <5a1f98566264a40895704d553e9acf8cfda0659c.1423499826.git.jpoimboe@redhat.com> <54DA55CA.3080408@suse.cz> <20150213160441.GG27180@treble.redhat.com> <20150213204924.GH27180@treble.redhat.com> User-Agent: Alpine 2.00 (LNX 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2336 Lines: 64 On Fri, 13 Feb 2015, Josh Poimboeuf wrote: > On Fri, Feb 13, 2015 at 05:17:10PM +0100, Miroslav Benes wrote: > > On Fri, 13 Feb 2015, Josh Poimboeuf wrote: > > > Hm, even with Jiri Slaby's suggested fix to add the completion to the > > > unregister path, I still get a lockdep warning. This looks more insidious, > > > related to the locking order of a kernfs lock and the klp lock. I'll need to > > > look at this some more... > > > > Yes, I was afraid of this. Lockdep warning is a separate bug. It is caused > > by taking klp_mutex in enabled_store. During rmmod klp_unregister_patch > > takes klp_mutex and destroys the sysfs structure. If somebody writes to > > enabled just after unregister takes the mutex and before the sysfs > > removal, he would cause the deadlock, because enabled_store takes the > > "sysfs lock" and then klp_mutex. That is exactly what the lockdep tells us > > below. > > > > We can look for inspiration elsewhere. Grep for s_active through git log > > of the mainline offers several commits which dealt exactly with this. Will > > browse through that... > > Thanks Miroslav, please let me know what you find. It wouldn't surprise > me if this were a very common problem. > > One option would be to move the enabled_store() work out to a workqueue > or something. Yes, that is one possibility. It is not the only one. 1. we could replace mutex_lock in enabled_store with mutex_trylock. If the lock was not acquired we would return -EBUSY. Or could we 'return restart_syscall' (maybe after some tiny msleep)? 2. we could reorganize klp_unregister_patch somehow and move sysfs removal out of mutex protection. Miroslav > > > > > > To recreate: > > > > > > insmod livepatch-sample.ko > > > > > > # wait for patching to complete > > > > > > ~/a.out & <-- simple program which opens the "enabled" file in the background > > > > I didn't even need such a program. Lockdep warned me with sole insmod, > > echo and rmmod. It is magically clever. > > Ah, even easier... lockdep is awesome. > > -- > Josh > -- Miroslav Benes SUSE Labs -- 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/