[PATCH] xfs: prevent race in zoned space reservations

[Hans Holmberg <hans.holmberg@wdc.com>]

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: 0bb2193056b5 ("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>
---
 fs/xfs/xfs_zone_space_resv.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/xfs/xfs_zone_space_resv.c b/fs/xfs/xfs_zone_space_resv.c
index 5c6e6ef627e4..7aa3c74fb2e0 100644
--- a/fs/xfs/xfs_zone_space_resv.c
+++ b/fs/xfs/xfs_zone_space_resv.c
@@ -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)
-- 
2.43.0