From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933275AbbGJUmw (ORCPT ); Fri, 10 Jul 2015 16:42:52 -0400 Received: from mx1.redhat.com ([209.132.183.28]:39095 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933190AbbGJUmo (ORCPT ); Fri, 10 Jul 2015 16:42:44 -0400 Date: Fri, 10 Jul 2015 22:42:40 +0200 (CEST) From: =?ISO-8859-15?Q?Luk=E1=A8_Czerner?= X-X-Sender: lczerner@localhost.localdomain To: Jan Kara cc: Dave Chinner , viro@ZenIV.linux.org.uk, bfields@fieldses.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [RFC][PATCH] fs: Prevent syncing frozen file system In-Reply-To: <20150710142525.GA31525@quack.suse.cz> Message-ID: References: <1436463945-12556-1-git-send-email-lczerner@redhat.com> <20150709234012.GI3902@dastard> <20150710142525.GA31525@quack.suse.cz> User-Agent: Alpine 2.00 (LFD 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 On Fri, 10 Jul 2015, Jan Kara wrote: > Date: Fri, 10 Jul 2015 16:25:25 +0200 > From: Jan Kara > To: Dave Chinner > Cc: Lukas Czerner , viro@ZenIV.linux.org.uk, > bfields@fieldses.org, linux-kernel@vger.kernel.org, > linux-fsdevel@vger.kernel.org > Subject: Re: [RFC][PATCH] fs: Prevent syncing frozen file system > > On Fri 10-07-15 09:40:12, Dave Chinner wrote: > > On Thu, Jul 09, 2015 at 07:45:45PM +0200, Lukas Czerner wrote: > > > Currently we can end up in a deadlock because of broken > > > sb_start_write -> s_umount ordering. > > > > > > The race goes like this: > > > > > > - write the file > > > - unlink the file - final_iput will not be calles as file is opened > > > - freeze the file system > > > - Now simultaneously close the file and call sync (or syncfs on that > > > particular file system). Sync will get to wait_sb_inodes() where it will > > > grab the referece to the inode (__iget()) and later to call iput(). > > > > This problem goes away with the sync scalability patchset that josef > > has been trying to get merged: > > > > git://git.kernel.org/pub/scm/linux/kernel/git/josef/btrfs-next.git superblock-scaling > > > > That patchset removes the full sb inodes list walk in > > wait_sb_inodes() and replaces it with a walk of inodes cleaned > > during the sync, which will be an empty list in the case of sync > > running on an empty filesystem. This commit does the work: > > > > https://git.kernel.org/cgit/linux/kernel/git/josef/btrfs-next.git/commit/?h=superblock-scaling&id=9bea30d5f4521db674203f365b1e0970588b2650 > > > > > that there are now several outstanding issues that it fixes in one > > go?> > > Not sure where that got stuck - oh, maybe on Tejun's memcg writeback series > which was clashing with it. Josef? > > > > If we manage to close the file and drop the reference in between those > > > calls sync will attempt to do a iput_final() because the inode is now > > > unlinked and we're holding the last reference to it. This will > > > however block on a frozen file system (ext4_delete_inode for > > > example). > > > > > > Note that I've not been able to reproduce the issue, I've only seen this > > > happen once. However with some instrumentation (like msleep() in the > > > wait_sb_inodes() it can be achieved. > > > > > > Fix this by properly doing sb_start_write/sb_end_write to prevent us > > > from fsfreeze. > > > > > > Note that with this patch syncfs will block on the frozen file system > > > which is probably ok, but sync will block if any file system happens to > > > be frozen - not sure if that's a problem, but it's certainly different > > > from what we've been used to. > > > > sync should not block on frozen fileystems. By definition, a frozen > > filesystem is a clean filesystem, and so sync should really just be > > skipping over them. > > Just for record I agree with Dave. Sync on frozen fs should just return. > And freeze protection in iterate_supers() looks just wrong. Sure, that's why it's rfc. Anyway with the change Dave mentioned the deadlock should not be possible anymore. However anywhere where we take s_umount before sb_start_write we could deadlock, so it might be worth adding a warning into sb_start_write() maybe ? -Lukas > > Honza > > > > +++ b/fs/super.c > > > @@ -514,10 +514,17 @@ void iterate_supers(void (*f)(struct super_block *, void *), void *arg) > > > sb->s_count++; > > > spin_unlock(&sb_lock); > > > > > > + /* > > > + * Whatever we're going to do to the file system we have to > > > + * make sure that we'll not end up blocking on frozen file > > > + * system. > > > + */ > > > + sb_start_write(sb); > > > down_read(&sb->s_umount); > > > if (sb->s_root && (sb->s_flags & MS_BORN)) > > > f(sb, arg); > > > up_read(&sb->s_umount); > > > + sb_end_write(sb); > > > > > > spin_lock(&sb_lock); > > > if (p) > > > > That deadlocks sysrq-j (emergency thaw)... > > > > Cheers, > > > > Dave. > > -- > > Dave Chinner > > david@fromorbit.com > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in > > the body of a message to majordomo@vger.kernel.org > > More majordomo info at http://vger.kernel.org/majordomo-info.html >