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

Aaron Tomlin
2014-09-17T20:48:49Z

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