Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752480AbbBQPz5 (ORCPT ); Tue, 17 Feb 2015 10:55:57 -0500 Received: from mx1.redhat.com ([209.132.183.28]:42717 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752269AbbBQPzz (ORCPT ); Tue, 17 Feb 2015 10:55:55 -0500 Date: Tue, 17 Feb 2015 09:55:47 -0600 From: Josh Poimboeuf To: Miroslav Benes 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 Message-ID: <20150217155547.GG11861@treble.redhat.com> References: <5a1f98566264a40895704d553e9acf8cfda0659c.1423499826.git.jpoimboe@redhat.com> <54DA55CA.3080408@suse.cz> <20150213160441.GG27180@treble.redhat.com> <20150213204924.GH27180@treble.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2483 Lines: 53 On Mon, Feb 16, 2015 at 05:06:15PM +0100, Miroslav Benes wrote: > 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)? Hm, doesn't that still violate the locking order rules? I thought locks always had to be taken in the same order -- always sysfs before klp, or klp before sysfs. Not sure if there would still be any deadlocks lurking, but lockdep might still complain. > 2. we could reorganize klp_unregister_patch somehow and move sysfs removal > out of mutex protection. Yeah, I was thinking about this too. Pretty sure we'd have to remove both the sysfs add and the sysfs removal from mutex protection. I like this option if we can get it to work. -- Josh -- 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/