From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933184Ab2GKUQH (ORCPT ); Wed, 11 Jul 2012 16:16:07 -0400 Received: from cantor2.suse.de ([195.135.220.15]:48722 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932456Ab2GKUQF (ORCPT ); Wed, 11 Jul 2012 16:16:05 -0400 Date: Wed, 11 Jul 2012 22:16:01 +0200 From: Jan Kara To: Jeff Moyer Cc: Jan Kara , LKML , linux-fsdevel@vger.kernel.org, Tejun Heo , Jens Axboe , mgalbraith@suse.com, Thomas Gleixner Subject: Re: Deadlocks due to per-process plugging Message-ID: <20120711201601.GB9779@quack.suse.cz> References: <20120711133735.GA8122@quack.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 On Wed 11-07-12 12:05:51, Jeff Moyer wrote: > Jan Kara writes: > > > Hello, > > > > we've recently hit a deadlock in our QA runs which is caused by the > > per-process plugging code. The problem is as follows: > > process A process B (kjournald) > > generic_file_aio_write() > > blk_start_plug(&plug); > > ... > > somewhere in here we allocate memory and > > direct reclaim submits buffer X for IO > > ... > > ext3_write_begin() > > ext3_journal_start() > > we need more space in a journal > > so we want to checkpoint old transactions, > > we block waiting for kjournald to commit > > a currently running transaction. > > journal_commit_transaction() > > wait for IO on buffer X > > to complete as it is part > > of the current transaction > > > > => deadlock since A waits for B and B waits for A to do unplug. > > BTW: I don't think this is really ext3/ext4 specific. I think other > > filesystems can get into problems as well when direct reclaim submits some > > IO and the process subsequently blocks without submitting the IO. > > So, I thought schedule would do the flush. Checking the code: > > asmlinkage void __sched schedule(void) > { > struct task_struct *tsk = current; > > sched_submit_work(tsk); > __schedule(); > } > > And sched_submit_work looks like this: > > static inline void sched_submit_work(struct task_struct *tsk) > { > if (!tsk->state || tsk_is_pi_blocked(tsk)) > return; > /* > * If we are going to sleep and we have plugged IO queued, > * make sure to submit it to avoid deadlocks. > */ > if (blk_needs_flush_plug(tsk)) > blk_schedule_flush_plug(tsk); > } > > This eventually ends in a call to blk_run_queue_async(q) after > submitting the I/O from the plug list. Right? So is the question > really why doesn't the kblockd workqueue get scheduled? Ah, I didn't know this. Thanks for the hint. So in the kdump I have I can see requests queued in tsk->plug despite the process is sleeping in TASK_UNINTERRUPTIBLE state. So the only way how unplug could have been omitted is if tsk_is_pi_blocked() was true. Rummaging through the dump... indeed task has pi_blocked_on = 0xffff8802717d79c8. The dump is from an -rt kernel (I just didn't originally thought that makes any difference) so actually any mutex is rtmutex and thus tsk_is_pi_blocked() is true whenever we are sleeping on a mutex. So this seems like a bug in rtmutex code. Thomas, you seemed to have added that condition... Any idea how to avoid the deadlock? Honza -- Jan Kara SUSE Labs, CR