From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E4AD5C5ACC4 for ; Thu, 20 Feb 2020 03:32:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C6380208C4 for ; Thu, 20 Feb 2020 03:32:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727601AbgBTDcY (ORCPT ); Wed, 19 Feb 2020 22:32:24 -0500 Received: from mail104.syd.optusnet.com.au ([211.29.132.246]:54917 "EHLO mail104.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727476AbgBTDcY (ORCPT ); Wed, 19 Feb 2020 22:32:24 -0500 Received: from dread.disaster.area (pa49-195-185-106.pa.nsw.optusnet.com.au [49.195.185.106]) by mail104.syd.optusnet.com.au (Postfix) with ESMTPS id E7D00820879; Thu, 20 Feb 2020 14:32:18 +1100 (AEDT) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1j4cZV-0005hM-9R; Thu, 20 Feb 2020 14:32:17 +1100 Date: Thu, 20 Feb 2020 14:32:17 +1100 From: Dave Chinner To: Brian Foster Cc: linux-xfs@vger.kernel.org Subject: Re: [PATCH v2] xfs: fix iclog release error check race with shutdown Message-ID: <20200220033217.GN10776@dread.disaster.area> References: <20200218175425.20598-1-bfoster@redhat.com> <20200218215243.GS10776@dread.disaster.area> <20200219131232.GA24157@bfoster> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200219131232.GA24157@bfoster> User-Agent: Mutt/1.10.1 (2018-07-13) X-Optus-CM-Score: 0 X-Optus-CM-Analysis: v=2.3 cv=W5xGqiek c=1 sm=1 tr=0 a=bkRQb8bsQZKWSSj4M57YXw==:117 a=bkRQb8bsQZKWSSj4M57YXw==:17 a=jpOVt7BSZ2e4Z31A5e1TngXxSK0=:19 a=kj9zAlcOel0A:10 a=l697ptgUJYAA:10 a=20KFwNOVAAAA:8 a=VwQbUJbxAAAA:8 a=7-415B0cAAAA:8 a=YLTrRmUPn5wXjQ3j8bMA:9 a=CjuIK1q_8ugA:10 a=AjGcO6oz07-iQ99wixmX:22 a=biEYGPWJfzWAr4FL6Ov7:22 Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Wed, Feb 19, 2020 at 08:12:32AM -0500, Brian Foster wrote: > On Wed, Feb 19, 2020 at 08:52:43AM +1100, Dave Chinner wrote: > > On Tue, Feb 18, 2020 at 12:54:25PM -0500, Brian Foster wrote: > > > Prior to commit df732b29c8 ("xfs: call xlog_state_release_iclog with > > > l_icloglock held"), xlog_state_release_iclog() always performed a > > > locked check of the iclog error state before proceeding into the > > > sync state processing code. As of this commit, part of > > > xlog_state_release_iclog() was open-coded into > > > xfs_log_release_iclog() and as a result the locked error state check > > > was lost. > > > > > > The lockless check still exists, but this doesn't account for the > > > possibility of a race with a shutdown being performed by another > > > task causing the iclog state to change while the original task waits > > > on ->l_icloglock. This has reproduced very rarely via generic/475 > > > and manifests as an assert failure in __xlog_state_release_iclog() > > > due to an unexpected iclog state. > > > > > > Restore the locked error state check in xlog_state_release_iclog() > > > to ensure that an iclog state update via shutdown doesn't race with > > > the iclog release state processing code. > > > > > > Fixes: df732b29c807 ("xfs: call xlog_state_release_iclog with l_icloglock held") > > > Reported-by: Zorro Lang > > > Signed-off-by: Brian Foster > > > --- > > > > > > v2: > > > - Include Fixes tag. > > > - Use common error path to include shutdown call. > > > v1: https://lore.kernel.org/linux-xfs/20200214181528.24046-1-bfoster@redhat.com/ > > > > > > fs/xfs/xfs_log.c | 13 +++++++++---- > > > 1 file changed, 9 insertions(+), 4 deletions(-) > > > > > > diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c > > > index f6006d94a581..796ff37d5bb5 100644 > > > --- a/fs/xfs/xfs_log.c > > > +++ b/fs/xfs/xfs_log.c > > > @@ -605,18 +605,23 @@ xfs_log_release_iclog( > > > struct xlog *log = mp->m_log; > > > bool sync; > > > > > > - if (iclog->ic_state == XLOG_STATE_IOERROR) { > > > - xfs_force_shutdown(mp, SHUTDOWN_LOG_IO_ERROR); > > > - return -EIO; > > > - } > > > > hmmmm. > > > > xfs_force_shutdown() will does nothing if the iclog at the head of > > the log->iclog list is marked with XLOG_STATE_IOERROR before IO is > > submitted. In general, that is the case here as the head iclog is > > what xlog_state_get_iclog_space() returns. > > > > i.e. XLOG_STATE_IOERROR here implies the log has already been shut > > down because the state is IOERROR rather than ACTIVE or WANT_SYNC as > > was set when the iclog was obtained from > > xlog_state_get_iclog_space(). > > > > I'm not sure that assumption is always true. If the iclog is (was) > WANT_SYNC, for example, then it's already been switched out from the > head of that list. Yes, but if the iclog is in XLOG_STATE_IOERROR here, the only way that state change can happen is if a shutdown has been completely processed between getting the iclog from xlog_state_get_iclog_space() and releasing it here. It can't come from an actual IO error, because a iclog being written to can't be under IO. Hence it doesn't matter if it was XLOG_STATE_ACTIVE at the head of the log or XLOG_STATE_WANT_SYNC and ready for IO; that state has been overwritten by the shutdown that has already been completed.... > That might only happen if a checkpoint happens to > align nicely with the end of an iclog, but that's certainly possible. We > also need to consider that ->l_icloglock cycles here and thus somebody > else could switch out the head iclog.. Yup, that can happen, but the only way that we can get to the XLOG_STATE_IOERROR here is for a shutdown to have already processed a shutdown. > > > > + if (iclog->ic_state == XLOG_STATE_IOERROR) > > > + goto error; > > > > > > if (atomic_dec_and_lock(&iclog->ic_refcnt, &log->l_icloglock)) { > > > + if (iclog->ic_state == XLOG_STATE_IOERROR) { > > > + spin_unlock(&log->l_icloglock); > > > + goto error; > > > + } > > > sync = __xlog_state_release_iclog(log, iclog); > > > spin_unlock(&log->l_icloglock); > > > if (sync) > > > xlog_sync(log, iclog); > > > } > > > return 0; > > > +error: > > > + xfs_force_shutdown(mp, SHUTDOWN_LOG_IO_ERROR); > > > + return -EIO; > > > > Hence I suspect that this should be ASSERT(XLOG_FORCED_SHUTDOWN(log)) > > just like is in xfs_log_force_umount() when this pre-existing log > > IO error condition is tripped over... > > > > I think this change is fundamentally correct based on the simple fact > that we only ever set XLOG_STATE_IOERROR in the shutdown path. That > said, the assert would be potentially racy against the shutdown path > without any ordering guarantee that the release path might see the iclog > state update prior to the log state update and lead to a potentially > false negative assert failure. I suspect this is why the shutdown call > might have been made from here in the first place (and partly why I > didn't bother with it in the restored locked state check). *nod* And if we change this to XLOG_FORCED_SHUTDOWN() checks, then the need for an assert check goes away, anyway... > Given the context of this patch (fixing a regression) and the practical > history of this code (and the annoying process of identifying and > tracking down the source of these kind of shutdown buglets), I'm > inclined to have this patch preserve the historical and relatively > proven behavior as much as possible and consider further cleanups > separately... If that's the case, then I don't think the shutdown call should be moved - the second XLOG_STATE_IOERROR occurs under the l_icloglock. Given that XLOG_STATE_IOERROR can only be set while holding the l_icloglock during shutdown processing, it makes no sense to recurse into shutdown here. Cheers, Dave. -- Dave Chinner david@fromorbit.com