Re: [PATCH] xfs: use dedicated log worker wq to avoid deadlock with cil wq

["Luis R. Rodriguez" <mcgrof@kernel.org>]

On Thu, Mar 16, 2017 at 12:17:34PM -0700, Darrick J. Wong wrote:
> On Thu, Mar 09, 2017 at 01:18:17PM -0500, Brian Foster wrote:
> > The log covering background task used to be part of the xfssyncd
> > workqueue. That workqueue was removed as of commit 5889608df ("xfs:
> > syncd workqueue is no more") and the associated work item scheduled
> > to the xfs-log wq. The latter is used for log buffer I/O completion.

That went in on v3.8.

<-- snip -->

> > In this scenario
> > both workqueues are deadlocked waiting on eachother.
> > 
> > Add a new workqueue specifically for the high level log covering and
> > ail pushing worker, as was the case prior to commit 5889608df.
> > 
> > Diagnosed-by: David Jeffery <djeffery@redhat.com>
> > Signed-off-by: Brian Foster <bfoster@redhat.com>

> > ---
> > 
> > Note that this seems difficult to reproduce in practice because I
> > believe it also relies on memory pressure. Otherwise, the xfs-log wq
> > rescuer thread may be available to unwind the deadlock.

Still, a dead lock is no bueno. No happy user can exist with a deadlock.

> I /think/ this looks ok?  But let me run it through xfstests before I
> commit to anything. :)

Provided that goes smooth -- this seems like a stable candidate to me.
Otherwise we're going to have to carry it anyway on distributions.

  Luis