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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>
This commit is contained in:
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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8
fs/xfs/xfs_zone_space_resv.c
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@ -85,13 +85,13 @@ xfs_zoned_add_available(
struct xfs_zone_info *zi = mp->m_zone_info;
struct xfs_zone_reservation *reservation;
if (list_empty_careful(&zi->zi_reclaim_reservations)) {
xfs_add_freecounter(mp, XC_FREE_RTAVAILABLE, count_fsb);
spin_lock(&zi->zi_reservation_lock);
xfs_add_freecounter(mp, XC_FREE_RTAVAILABLE, count_fsb);
if (list_empty(&zi->zi_reclaim_reservations)) {
spin_unlock(&zi->zi_reservation_lock);
return;
}
spin_lock(&zi->zi_reservation_lock);
xfs_add_freecounter(mp, XC_FREE_RTAVAILABLE, count_fsb);
count_fsb = xfs_sum_freecounter(mp, XC_FREE_RTAVAILABLE);
list_for_each_entry(reservation, &zi->zi_reclaim_reservations, entry) {
if (reservation->count_fsb > count_fsb)