workqueue: skip nr_running sanity check in worker_enter_idle() if trustee is active

commit 544ecf310f0e7f51fa057ac2a295fc1b3b35a9d3 upstream.

worker_enter_idle() has WARN_ON_ONCE() which triggers if nr_running
isn't zero when every worker is idle.  This can trigger spuriously
while a cpu is going down due to the way trustee sets %WORKER_ROGUE
and zaps nr_running.

It first sets %WORKER_ROGUE on all workers without updating
nr_running, releases gcwq->lock, schedules, regrabs gcwq->lock and
then zaps nr_running.  If the last running worker enters idle
inbetween, it would see stale nr_running which hasn't been zapped yet
and trigger the WARN_ON_ONCE().

Fix it by performing the sanity check iff the trustee is idle.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index bb425b1..7947e16 100644 (file)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1215,8 +1215,13 @@ static void worker_enter_idle(struct worker *worker)
        } else
                wake_up_all(&gcwq->trustee_wait);
 
-       /* sanity check nr_running */
-       WARN_ON_ONCE(gcwq->nr_workers == gcwq->nr_idle &&
+       /*
+        * Sanity check nr_running.  Because trustee releases gcwq->lock
+        * between setting %WORKER_ROGUE and zapping nr_running, the
+        * warning may trigger spuriously.  Check iff trustee is idle.
+        */
+       WARN_ON_ONCE(gcwq->trustee_state == TRUSTEE_DONE &&
+                    gcwq->nr_workers == gcwq->nr_idle &&
                     atomic_read(get_gcwq_nr_running(gcwq->cpu)));
 }