[PATCH 4.19-rt] workqueue: Fix deadlock due to recursive locking of pool->lock

[Brennan Lamoreaux (VMware)]

Upstream commit d8bb65ab70f7 ("workqueue: Use rcuwait for wq_manager_wait")
replaced the waitqueue with rcuwait in the workqueue code. This change
involved removing the acquisition of pool->lock in put_unbound_pool(),
as it also adds the function wq_manager_inactive() which acquires this same
lock and is called one line later as a parameter to rcu_wait_event().

However, the backport of this commit in the PREEMPT_RT patchset
4.19.255-rt114 (patch 347) missed the removal of the acquisition of
pool->lock in put_unbound_pool(). This leads to a deadlock due to
recursive locking of pool->lock, as shown below in lockdep:

[  252.083713] WARNING: possible recursive locking detected
[  252.083718] 4.19.269-3.ph3-rt #1-photon Not tainted
[  252.083721] --------------------------------------------
[  252.083733] kworker/2:0/33 is trying to acquire lock:
[  252.083747] 000000000b7b1ceb (&pool->lock/1){....}, at:
put_unbound_pool+0x10d/0x260

[  252.083857]
               but task is already holding lock:
[  252.083860] 000000000b7b1ceb (&pool->lock/1){....}, at:
put_unbound_pool+0xbd/0x260

[  252.083876]
               other info that might help us debug this:
[  252.083897]  Possible unsafe locking scenario:

[  252.083900]        CPU0
[  252.083903]        ----
[  252.083904]   lock(&pool->lock/1);
[  252.083911]   lock(&pool->lock/1);
[  252.083919]
                *** DEADLOCK ***

[  252.083921]  May be due to missing lock nesting notation

Fix this deadlock by removing the pool->lock acquisition in
put_unbound_pool().

Signed-off-by: Brennan Lamoreaux (VMware) <brennanlamoreaux@xxxxxxxxx>
Cc: Daniel Wagner <wagi@xxxxxxxxx>
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
---
 kernel/workqueue.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index a9f3cc02bdc1..55ebdd56a5de 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3394,7 +3394,6 @@ static void put_unbound_pool(struct worker_pool *pool)
 	 * Because of how wq_manager_inactive() works, we will hold the
 	 * spinlock after a successful wait.
 	 */
-	raw_spin_lock_irq(&pool->lock);
 	rcuwait_wait_event(&manager_wait, wq_manager_inactive(pool),
 			   TASK_UNINTERRUPTIBLE);
 	pool->flags |= POOL_MANAGER_ACTIVE;
-- 
2.35.6