From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756008AbbJIIDm (ORCPT ); Fri, 9 Oct 2015 04:03:42 -0400 Received: from mail-wi0-f174.google.com ([209.85.212.174]:36073 "EHLO mail-wi0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755891AbbJIIDf (ORCPT ); Fri, 9 Oct 2015 04:03:35 -0400 Subject: Re: [RFC PATCH 1/2] ext4: Fix possible deadlock with local interrupts disabled and page-draining IPI To: Hillf Danton References: <062501d10262$d40d0a50$7c271ef0$@alibaba-inc.com> <56176C10.8040709@kyup.com> <062801d10265$5a749fc0$0f5ddf40$@alibaba-inc.com> Cc: "'linux-kernel'" , "Theodore Ts'o" , Andreas Dilger , linux-fsdevel@vger.kernel.org, SiteGround Operations , vbabka@suse.cz, gilad@benyossef.com, mgorman@suse.de, linux-mm@kvack.org, Marian Marinov From: Nikolay Borisov Message-ID: <561774D2.3050002@kyup.com> Date: Fri, 9 Oct 2015 11:03:30 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 MIME-Version: 1.0 In-Reply-To: <062801d10265$5a749fc0$0f5ddf40$@alibaba-inc.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/09/2015 10:37 AM, Hillf Danton wrote: >>>> @@ -109,8 +109,8 @@ static void ext4_finish_bio(struct bio *bio) >>>> if (bio->bi_error) >>>> buffer_io_error(bh); >>>> } while ((bh = bh->b_this_page) != head); >>>> - bit_spin_unlock(BH_Uptodate_Lock, &head->b_state); >>>> local_irq_restore(flags); >>> >>> What if it takes 100ms to unlock after IRQ restored? >> >> I'm not sure I understand in what direction you are going? Care to >> elaborate? >> > Your change introduces extra time cost the lock waiter has to pay in > the case that irq happens before the lock is released. [CC filesystem and mm people. For reference the thread starts here: http://thread.gmane.org/gmane.linux.kernel/2056996 ] Right, I see what you mean and it's a good point but when doing the patches I was striving for correctness and starting a discussion, hence the RFC. In any case I'd personally choose correctness over performance always ;). As I'm not an fs/ext4 expert and have added the relevant parties (please use reply-all from now on so that the thread is not being cut in the middle) who will be able to say whether it impact is going to be that big. I guess in this particular code path worrying about this is prudent as writeback sounds like a heavily used path. Maybe the problem should be approached from a different angle e.g. drain_all_pages and its reliance on the fact that the IPI will always be delivered in some finite amount of time? But what if a cpu with disabled interrupts is waiting on the task issuing the IPI? > >>>> + bit_spin_unlock(BH_Uptodate_Lock, &head->b_state); >>>> if (!under_io) { >>>> #ifdef CONFIG_EXT4_FS_ENCRYPTION >>>> if (ctx) >>>> -- >>>> 2.5.0 >>> >