[PATCH v2] workqueue: Add pool_workqueue to pending_pwqs list when unplugging multiple inactive works

[Matthew Brost <matthew.brost@intel.com>]

In unplug_oldest_pwq(), the first inactive work item on the
pool_workqueue is activated correctly. However, if multiple inactive
works exist on the same pool_workqueue, subsequent works fail to
activate because wq_node_nr_active.pending_pwqs is empty — the list
insertion is skipped when the pool_workqueue is plugged.

Fix this by checking for additional inactive works in
unplug_oldest_pwq() and updating wq_node_nr_active.pending_pwqs
accordingly.

v2:
 - Use pwq_activate_first_inactive(pwq, false) rather than open coding
   list operations (Tejun)

Cc: Carlos Santa <carlos.santa@intel.com>
Cc: Ryan Neph <ryanneph@google.com>
Cc: stable@vger.kernel.org
Cc: Tejun Heo <tj@kernel.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Waiman Long <longman@redhat.com>
Cc: linux-kernel@vger.kernel.org
Fixes: 4c065dbce1e8 ("workqueue: Enable unbound cpumask update on ordered workqueues")
Signed-off-by: Matthew Brost <matthew.brost@intel.com>

---

This bug was first reported by Google, where the Xe driver appeared to
hang due to a fencing signal not completing. We traced the issue to work
items not being scheduled, and it can be trivially reproduced on drm-tip
with the following commands:

shell0:
for i in {1..100}; do echo "Run $i"; xe_exec_threads --r \
threads-rebind-bindexecqueue; done

shell1:
for i in {1..1000}; do echo "toggle $i"; echo f > \
/sys/devices/virtual/workqueue/cpumask; echo ff > \
/sys/devices/virtual/workqueue/cpumask; echo fff > \
/sys/devices/virtual/workqueue/cpumask ; echo ffff > \
/sys/devices/virtual/workqueue/cpumask; sleep .1; done
---
 kernel/workqueue.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index b77119d71641..bee3f37fffde 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1849,8 +1849,17 @@ static void unplug_oldest_pwq(struct workqueue_struct *wq)
 	raw_spin_lock_irq(&pwq->pool->lock);
 	if (pwq->plugged) {
 		pwq->plugged = false;
-		if (pwq_activate_first_inactive(pwq, true))
+		if (pwq_activate_first_inactive(pwq, true)) {
+			/*
+			 * pwq is unbound. Additional inactive work_items need
+			 * to reinsert the pwq into nna->pending_pwqs, which
+			 * was skipped while pwq->plugged was true. See
+			 * pwq_tryinc_nr_active() for additional details.
+			 */
+			pwq_activate_first_inactive(pwq, false);
+
 			kick_pool(pwq->pool);
+		}
 	}
 	raw_spin_unlock_irq(&pwq->pool->lock);
 }
-- 
2.34.1