Re: [RFC PATCH] fs: Use a seperate wq for do_sync_work() to avoid a potential deadlock

Dave Chinner
2014-09-17T21:16:38Z

On Wed, Sep 17, 2014 at 09:46:35PM +0100, Aaron Tomlin wrote:
> 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.

What kernel did you see this deadlock on?

I don't see a deadlock here on a mainline kernel. The emergency sync
work blocks, the new work gets queued, and the workqueue
infrastructure simply pulls another kworker thread from the pool and
runs the new work. IOWs, I can't see how this would deadlock unless
the system_wq kworker pool has been fully depleted it's defined
per-cpu concurrency depth. If the kworker thread pool is depleted
then you have bigger problems than emergency sync not
deadlocking....

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com