Re: [PATCH 04/11] writeback: switch to per-bdi threads forflushing data

From: Jan Kara
Date: Thu May 28 2009 - 10:46:00 EST

On Wed 27-05-09 19:50:19, Jens Axboe wrote:
> > > +static int bdi_forker_task(void *ptr)
> > > +{
> > > + struct backing_dev_info *me = ptr;
> > > + DEFINE_WAIT(wait);
> > > +
> > > + for (;;) {
> > > + struct backing_dev_info *bdi, *tmp;
> > > +
> > > + /*
> > > + * Do this periodically, like kupdated() did before.
> > > + */
> > > + sync_supers();
> > Ugh, this looks nasty. Moreover I'm afraid of forker_task() getting stuck
> > (and thus not being able to start new threads) in sync_supers() when some
> > fs is busy and other needs to create flusher thread...
> > Why not just having a separate thread for this? I know we have lots of
> > kernel threads already but this one seems like a useful one... Or do you
> > plan getting rid of this completely sometime in the near future and sync
> > supers also from per-bdi thread (which would make a lot of sence to me)?
> It's ugly, and I think this is precisely what Ted hit. He's in umount,
> has ->s_umount sem held and waiting for IO.
I've looked into this a bit more because it was still nagging in the back
of my mind and I think there indeed is a race (although your sync writeback
waiting has now hidden it). The problem is following:
bdi flusher threads lives independently of filesystem being mounted or
not. So it can happen that bdi_kupdate() or bdi_pdflush() runs in parallel
with umount running in another thread. That should not really happen
1) umount can fail with EBUSY because generic_sync_bdi_inodes() holds
a reference to inode
2) we race more subtly and we get to call __writeback_single_inode() after
the filesystem has been unmounted (put_super() has been called).

So I believe you simply have to deal with superblock references and
umount semaphore in your patches...

> So there's definitely trouble brewing there. As a short term solution, a
> separate thread will do. Longer term, the sync_supers_bdi() type setup I
> mentioned earlier would probably be the best. But once we start dealing
> with the super blocks, we have to be more careful with referencing.
> Which we also discussed in a previous mail :-)

Jan Kara <jack@xxxxxxx>
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