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Fix snapshot automount expiry cancellation deadlock

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A deadlock occurs when snapshot expiry tasks are cancelled while holding
locks. The snapshot expiry task (snapentry_expire) spawns an umount
process and waits for it to complete. Concurrently, ARC memory pressure
triggers arc_prune which calls zfs_exit_fs(), attempting to cancel the
expiry task while holding locks. The umount process spawned by the
expiry task blocks trying to acquire locks held by arc_prune, which is
blocked waiting for the expiry task to complete. This creates a circular
dependency: expiry task waits for umount, umount waits for arc_prune,
arc_prune waits for expiry task.

Fix by adding non-blocking cancellation support to taskq_cancel_id().
The zfs_exit_fs() path calls zfsctl_snapshot_unmount_delay() to
reschedule the unmount, which needs to cancel any existing expiry task.
It now uses non-blocking cancellation to avoid waiting while holding
locks, breaking the deadlock by returning immediately when the task is
already running.

The per-entry se_taskqid_lock has been removed, with all taskqid
operations now protected by the global zfs_snapshot_lock held as
WRITER. Additionally, an se_in_umount flag prevents recursive waits when
zfsctl_destroy() is called during unmount. The taskqid is now only
cleared by the caller on successful cancellation; running tasks clear
their own taskqid upon completion.

Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #17941
This commit is contained in:
Ameer Hamza
2025-12-02 03:43:42 +05:00
committed by Brian Behlendorf
parent 663dc86de2
commit 74bbdda1ef
12 changed files with 69 additions and 48 deletions
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module/zfs/spa.c
+3 -3
View File
@@ -1934,7 +1934,7 @@ spa_deactivate(spa_t *spa)
list_destroy(&spa->spa_evicting_os_list);
list_destroy(&spa->spa_state_dirty_list);
taskq_cancel_id(system_delay_taskq, spa->spa_deadman_tqid);
taskq_cancel_id(system_delay_taskq, spa->spa_deadman_tqid, B_TRUE);
for (int t = 0; t < ZIO_TYPES; t++) {
for (int q = 0; q < ZIO_TASKQ_TYPES; q++) {
@@ -10451,7 +10451,7 @@ spa_sync(spa_t *spa, uint64_t txg)
spa->spa_sync_starttime = gethrtime();
taskq_cancel_id(system_delay_taskq, spa->spa_deadman_tqid);
taskq_cancel_id(system_delay_taskq, spa->spa_deadman_tqid, B_TRUE);
spa->spa_deadman_tqid = taskq_dispatch_delay(system_delay_taskq,
spa_deadman, spa, TQ_SLEEP, ddi_get_lbolt() +
NSEC_TO_TICK(spa->spa_deadman_synctime));
@@ -10508,7 +10508,7 @@ spa_sync(spa_t *spa, uint64_t txg)
spa_sync_rewrite_vdev_config(spa, tx);
dmu_tx_commit(tx);
taskq_cancel_id(system_delay_taskq, spa->spa_deadman_tqid);
taskq_cancel_id(system_delay_taskq, spa->spa_deadman_tqid, B_TRUE);
spa->spa_deadman_tqid = 0;
/*