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

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>
Cc: stable@vger.kernel.org

1 files changed, 7 insertions, 2 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 211eadb23323..c36c86cf7900 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1213,8 +1213,13 @@ 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)));
 }