linux-kernel - Re: [PATCH v7 4/4] workqueue: Unbind kworkers before sending them to exit()

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Message-ID: <Y73KeZ3g0WdukMQo@slm.duckdns.org>
Date:   Tue, 10 Jan 2023 10:28:41 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Phil Auld <pauld@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH v7 4/4] workqueue: Unbind kworkers before sending them to
 exit()

Hello,

The series generally looks good to me. Just one thing.

On Mon, Jan 09, 2023 at 01:33:16PM +0000, Valentin Schneider wrote:
> @@ -3658,13 +3702,24 @@ static void put_unbound_pool(struct worker_pool *pool)
>  			   TASK_UNINTERRUPTIBLE);
>  	pool->flags |= POOL_MANAGER_ACTIVE;
>  
> +	/*
> +	 * We need to hold wq_pool_attach_mutex() while destroying the workers,
> +	 * but we can't grab it in rcuwait_wait_event() as it can clobber
> +	 * current's task state. We can drop pool->lock here as we've set
> +	 * POOL_MANAGER_ACTIVE, no one else can steal our manager position.
> +	 */
> +	raw_spin_unlock_irq(&pool->lock);
> +	mutex_lock(&wq_pool_attach_mutex);
> +	raw_spin_lock_irq(&pool->lock);

The original pattern was a bit weird to begin with and this makes it quite
worse. Let's do something more straight forward like:

        while (true) {
                rcuwait_wait_event(&manager_wait,
                                   !(pool->flags & POOL_MANAGER_ACTIVE),
                                   TASK_UNINTERRUPTIBLE);
                mutex_lock(&wq_pool_attach_mutex);
                raw_spin_lock_irq(&pool->lock);
                if (!(pool->flags & POOL_MANAGER_ACTIVE)) {
                        pool->flags |= POOL_MANAGER_ACTIVE;
                        break;
                }
                raw_spin_unlock_irq(&pool->lock);
                mutex_unlock(&wq_pool_attach_mutex);
        }

Thanks.

-- 
tejun