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workqueue: Fix false positive stall reports

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On weakly ordered architectures (e.g., arm64), the lockless check in
wq_watchdog_timer_fn() can observe a reordering between the worklist
insertion and the last_progress_ts update. Specifically, the watchdog
can see a non-empty worklist (from a list_add) while reading a stale
last_progress_ts value, causing a false positive stall report.

This was confirmed by reading pool->last_progress_ts again after holding
pool->lock in wq_watchdog_timer_fn():

  workqueue watchdog: pool 7 false positive detected!
    lockless_ts=4784580465 locked_ts=4785033728
    diff=453263ms worklist_empty=0

To avoid slowing down the hot path (queue_work, etc.), recheck
last_progress_ts with pool->lock held. This will eliminate the false
positive with minimal overhead.

Remove two extra empty lines in wq_watchdog_timer_fn() as we are on it.

Fixes: 82607adcf9 ("workqueue: implement lockup detector")
Cc: stable@vger.kernel.org # v4.5+
Assisted-by: claude-code:claude-opus-4-6
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Song Liu
2026-03-21 20:30:45 -07:00
committed by Tejun Heo
parent 98c790b100
commit c7f27a8ab9
1 changed files with 21 additions and 3 deletions
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24
kernel/workqueue.c
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@@ -7699,8 +7699,28 @@ static void wq_watchdog_timer_fn(struct timer_list *unused)
else
ts = touched;
/* did we stall? */
/*
* Did we stall?
*
* Do a lockless check first. On weakly ordered
* architectures, the lockless check can observe a
* reordering between worklist insert_work() and
* last_progress_ts update from __queue_work(). Since
* __queue_work() is a much hotter path than the timer
* function, we handle false positive here by reading
* last_progress_ts again with pool->lock held.
*/
if (time_after(now, ts + thresh)) {
scoped_guard(raw_spinlock_irqsave, &pool->lock) {
pool_ts = pool->last_progress_ts;
if (time_after(pool_ts, touched))
ts = pool_ts;
else
ts = touched;
}
if (!time_after(now, ts + thresh))
continue;
lockup_detected = true;
stall_time = jiffies_to_msecs(now - pool_ts) / 1000;
max_stall_time = max(max_stall_time, stall_time);
@@ -7712,8 +7732,6 @@ static void wq_watchdog_timer_fn(struct timer_list *unused)
pr_cont_pool_info(pool);
pr_cont(" stuck for %us!\n", stall_time);
}
}
if (lockup_detected)