On Thu, Nov 13, 2014 at 02:24:00PM -0500, Brian Foster wrote:
> The xfslogd workqueue is a global, single-job workqueue for buffer ioend
> processing. This means we allow for a single work item at a time for all
> possible XFS mounts on a system. fsstress testing in loopback XFS over
> XFS configurations has reproduced xfslogd deadlocks due to the single
> threaded nature of the queue and dependencies introduced between the
> separate XFS instances by online discard (-o discard).
> Discard over a loopback device converts the discard request to a hole
> punch (fallocate) on the underlying file. Online discard requests are
> issued synchronously and from xfslogd context in XFS, hence the xfslogd
> workqueue is blocked in the upper fs waiting on a hole punch request to
> be servied in the lower fs. If the lower fs issues I/O that depends on
> xfslogd to complete, both filesystems end up hung indefinitely. This is
> reproduced reliabily by generic/013 on XFS->loop->XFS test devices with
> the '-o discard' mount option.
> Further, docker implementations appear to use this kind of configuration
> for container instance filesystems by default (container fs->dm->
> loop->base fs) and therefore are subject to this deadlock when running
> on XFS.
> Replace the global xfslogd workqueue with a per-mount variant. This
> guarantees each mount access to a single worker and prevents deadlocks
> due to inter-fs dependencies introduced by discard. Since the queue is
> only responsible for buffer iodone processing at this point in time,
> rename xfslogd to xfs-buf.
> Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
Looks good. I'll take this as is and we can refine the way we point
to the workqueue in the patches that separate the log buffer