[Ocfs2-devel] [Ocfs2-dev] BUG: deadlock with umount and ocfs2 workqueue triggered by ocfs2rec thread

[Jan Kara]

On Wed 17-01-18 16:21:35, Jan Kara wrote:
> Hello,
> 
> On Fri 12-01-18 16:25:56, Eric Ren wrote:
> > On 01/12/2018 11:43 AM, Shichangkuo wrote:
> > > Hi all,
> > > 　　Now we are testing ocfs2 with 4.14 kernel, and we finding a deadlock with umount and ocfs2 workqueue triggered by ocfs2rec thread. The stack as follows:
> > > journal recovery work:
> > > [<ffffffff8a8c0694>] call_rwsem_down_read_failed+0x14/0x30
> > > [<ffffffffc0d5d652>] ocfs2_finish_quota_recovery+0x62/0x450 [ocfs2]
> > > [<ffffffffc0d21221>] ocfs2_complete_recovery+0xc1/0x440 [ocfs2]
> > > [<ffffffff8a09a1f0>] process_one_work+0x130/0x350
> > > [<ffffffff8a09a946>] worker_thread+0x46/0x3b0
> > > [<ffffffff8a0a0e51>] kthread+0x101/0x140
> > > [<ffffffff8aa002ff>] ret_from_fork+0x1f/0x30
> > > [<ffffffffffffffff>] 0xffffffffffffffff
> > > 
> > > /bin/umount:
> > > [<ffffffff8a099b24>] flush_workqueue+0x104/0x3e0
> > > [<ffffffffc0cf18db>] ocfs2_truncate_log_shutdown+0x3b/0xc0 [ocfs2]
> > > [<ffffffffc0d4fd6c>] ocfs2_dismount_volume+0x8c/0x3d0 [ocfs2]
> > > [<ffffffffc0d500e1>] ocfs2_put_super+0x31/0xa0 [ocfs2]
> > > [<ffffffff8a2445bd>] generic_shutdown_super+0x6d/0x120
> > > [<ffffffff8a24469d>] kill_block_super+0x2d/0x60
> > > [<ffffffff8a244e71>] deactivate_locked_super+0x51/0x90
> > > [<ffffffff8a263a1b>] cleanup_mnt+0x3b/0x70
> > > [<ffffffff8a09e9c6>] task_work_run+0x86/0xa0
> > > [<ffffffff8a003d70>] exit_to_usermode_loop+0x6d/0xa9
> > > [<ffffffff8a003a2d>] do_syscall_64+0x11d/0x130
> > > [<ffffffff8aa00113>] entry_SYSCALL64_slow_path+0x25/0x25
> > > [<ffffffffffffffff>] 0xffffffffffffffff
> > > 　　
> > > Function ocfs2_finish_quota_recovery try to get sb->s_umount, which was already locked by umount thread, then get a deadlock.
> > 
> > Good catch, thanks for reporting.  Is it reproducible? Can you please share
> > the steps for reproducing this issue?
> > > This issue was introduced by c3b004460d77bf3f980d877be539016f2df4df12 and 5f530de63cfc6ca8571cbdf58af63fb166cc6517.
> > > I think we cannot use :: s_umount, but the mutex ::dqonoff_mutex was already removed.
> > > Shall we add a new mutex?
> > 
> > @Jan, I don't look into the code yet, could you help me understand why we
> > need to get sb->s_umount in ocfs2_finish_quota_recovery?
> > Is it because that the quota recovery process will start at umounting? or
> > some where else?
> 
> I was refreshing my memory wrt how ocfs2 quota recovery works. The problem
> is the following: We load information about all quota information that
> needs recovering (this is possibly for other nodes) in
> ocfs2_begin_quota_recovery() that gets called during mount. Real quota
> recovery happens from the recovery thread in ocfs2_finish_quota_recovery().
> We need to protect code running there from dquot_disable() calls as that
> will free structures we use for updating quota information etc. Currently
> we use sb->s_umount for that protection.
> 
> The problem above apparently happens when someone calls umount before the
> recovery thread can finish quota recovery. I will think more about how to
> fix the locking so that this lock inversion does not happen...

So could we move ocfs2_recovery_exit() call in ocfs2_dismount_volume() up
before ocfs2_disable_quotas()? It seems possible to me, I'm just not sure
if there are not some hidden dependencies on recovery being shut down only
after truncate log / local alloc. If we can do that, we could remove
s_umount protection from ocfs2_finish_quota_recovery() and thus resolve the
race.

								Honza
-- 
Jan Kara <jack at suse.com>
SUSE Labs, CR