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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/os/freebsd/spl/spl_taskq.c
+10 -3
View File
@@ -351,7 +351,7 @@ taskq_free(taskq_ent_t *task)
}
int
taskq_cancel_id(taskq_t *tq, taskqid_t tid)
taskq_cancel_id(taskq_t *tq, taskqid_t tid, boolean_t wait)
{
uint32_t pend;
int rc;
@@ -362,12 +362,12 @@ taskq_cancel_id(taskq_t *tq, taskqid_t tid)
if (ent->tqent_type == NORMAL_TASK) {
rc = taskqueue_cancel(tq->tq_queue, &ent->tqent_task, &pend);
if (rc == EBUSY)
if (rc == EBUSY && wait)
taskqueue_drain(tq->tq_queue, &ent->tqent_task);
} else {
rc = taskqueue_cancel_timeout(tq->tq_queue,
&ent->tqent_timeout_task, &pend);
if (rc == EBUSY) {
if (rc == EBUSY && wait) {
taskqueue_drain_timeout(tq->tq_queue,
&ent->tqent_timeout_task);
}
@@ -381,6 +381,13 @@ taskq_cancel_id(taskq_t *tq, taskqid_t tid)
}
/* Free the extra reference we added with taskq_lookup. */
taskq_free(ent);
/*
* If task was running and we didn't wait, return EBUSY.
* Otherwise return 0 if cancelled or ENOENT if not found.
*/
if (rc == EBUSY && !wait)
return (EBUSY);
return (pend ? 0 : ENOENT);
}