From: Aaron Tomlin <atomlin@redhat.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk,
david@fromorbit.com, bmr@redhat.com, jcastillo@redhat.com,
mguzik@redhat.com, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] fs: Use a seperate wq for do_sync_work() to avoid a potential deadlock
Date: Wed, 17 Sep 2014 21:46:35 +0100 [thread overview]
Message-ID: <20140917204634.GB25400@atomlin.usersys.redhat.com> (raw)
In-Reply-To: <20140917182202.GE19308@redhat.com>
On Wed, Sep 17, 2014 at 08:22:02PM +0200, Oleg Nesterov wrote:
> On 09/17, Aaron Tomlin wrote:
> >
> > Since do_sync_work() is a deferred function it can block indefinitely by
> > design. At present do_sync_work() is added to the global system_wq.
> > As such a deadlock is theoretically possible between sys_unmount() and
> > sync_filesystems():
> >
> > * The current work fn on the system_wq (do_sync_work()) is blocked
> > waiting to aquire a sb's s_umount for reading.
> >
> > * The "umount" task is the current owner of the s_umount in
> > question but is waiting for do_sync_work() to continue.
> > Thus we hit a deadlock situation.
> >
> I can't comment the patches in this area, but I am just curious...
>
> Could you explain this deadlock in more details? I simply can't understand
> what "waiting for do_sync_work()" actually means.
Hopefully this helps:
"umount" "events/1"
sys_umount sysrq_handle_sync
deactivate_super(sb) emergency_sync
{ schedule_work(work)
... queue_work(system_wq, work)
down_write(&s->s_umount) do_sync_work(work)
... sync_filesystems(0)
kill_block_super(s) ...
generic_shutdown_super(sb) down_read(&sb->s_umount)
// sop->put_super(sb)
ext4_put_super(sb)
invalidate_bdev(sb->s_bdev)
lru_add_drain_all()
for_each_online_cpu(cpu) {
schedule_work_on(cpu, work)
queue_work_on(cpu, system_wq, work)
...
}
}
- Both lru_add_drain and do_sync_work work items are added to
the same global system_wq
- The current work fn on the system_wq is do_sync_work and is
blocked waiting to aquire an sb's s_umount for reading
- The umount task is the current owner of the s_umount in
question but is waiting for do_sync_work to continue.
Thus we hit a deadlock situation.
--
Aaron Tomlin
next prev parent reply other threads:[~2014-09-17 20:48 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-09-17 11:39 [RFC PATCH] fs: Use a seperate wq for do_sync_work() to avoid a potential deadlock Aaron Tomlin
2014-09-17 18:22 ` Oleg Nesterov
2014-09-17 20:46 ` Aaron Tomlin [this message]
2014-09-17 21:16 ` Dave Chinner
2014-09-19 15:44 ` Aaron Tomlin
2014-09-17 21:42 ` Oleg Nesterov
2014-09-19 9:35 ` Aaron Tomlin
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