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

["Srivatsa S. Bhat" <srivatsa@csail.mit.edu>]

On 2/28/23 2:49 PM, Brennan Lamoreaux (VMware) wrote:
> 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@gmail.com>
> Cc: Daniel Wagner <wagi@monom.org>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: Tejun Heo <tj@kernel.org>

Reviewed-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>

> ---
>  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;
> 

 
Regards,
Srivatsa
VMware Photon OS