[PATCH] workqueue: Fix false positive stall reports

[Song Liu <song@kernel.org>]

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.

Assisted-by: claude-code:claude-opus-4-6
Signed-off-by: Song Liu <song@kernel.org>
---
 kernel/workqueue.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index b77119d71641..5b501ff1223a 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -7701,6 +7701,23 @@ static void wq_watchdog_timer_fn(struct timer_list *unused)
 
 		/* did we stall? */
 		if (time_after(now, ts + thresh)) {
+			unsigned long irq_flags;
+
+			raw_spin_lock_irqsave(&pool->lock, irq_flags);
+			/*
+			 * Recheck last_progress_ts with pool->lock, this
+			 * eliminates false positive where we report wq
+			 * stall for newly queued work.
+			 */
+			pool_ts = READ_ONCE(pool->last_progress_ts);
+			if (time_after(pool_ts, touched))
+				ts = pool_ts;
+			else
+				ts = touched;
+			raw_spin_unlock_irqrestore(&pool->lock, irq_flags);
+			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 +7729,6 @@ 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)
-- 
2.52.0