xfs: don't let racing writers consume the free zones reserved for GC

xfs_try_open_zone() checks the free zone count to avoid grabbing zones
required for GC forward progress, but does so without decreasing the
counter, letting multiple user writers through to call xfs_open_zone()
and collectively gobble up all free zones, stalling gc and eventually
leading to user threads getting stuck waiting for free space.

Fold the check and the accounting into a single atomic claim in
xfs_open_zone() instead.  The free zone count is decremented before the
zone is looked up and given back if none could be grabbed, so the reserve
can no longer be raced away.  User allocations only claim a zone if that
leaves the reserve behind, while GC just needs one free zone and thus can
still use the zones that are kept for it. Also add a warning if we end
up not finding a free zone in the xarray, as this should never happen.

Fixes: 4e4d52075577 ("xfs: add the zoned space allocator")
Signed-off-by: Hans Holmberg <hans.holmberg@wdc.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

0 files changed, 0 insertions, 0 deletions