xfs: prevent race in zoned space reservations

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xfs_zoned_add_available() checks whether the reservation list is empty
before adding blocks to the available-space counter.  This check is not
serialized against a task adding itself to the reservation list however.

This allows the space provider to observe an empty list, after which a
reserver can enqueue itself and retry the counter before the new space is
added.  The provider then adds the space and returns without waking the
now-eligible reserver, leaving it asleep until GC or another event
provides a wakeup, potentially adding seconds to max write latency.

Take the reservation lock before updating the counter and checking the
list.  Use list_empty() because the list is now inspected under its lock.

Taking a per-mount lock when handing back space is far from ideal, but
benchmarking with null_blk showed no measurable performance regression.

Fixes: 0bb2193056 ("xfs: add support for zoned space reservations")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260609075655.1698743-1-hch@lst.de?part=2
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>

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Hans Holmberg 2026-08-26 14:32:19 +02:00 • committed by Carlos Maiolino

parent f789d291bb

commit ff99a5f6cb

1 changed files with 4 additions and 4 deletions

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fs/xfs/xfs_zone_space_resv.c

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