linux-kernel - Re: [PATCH v6 4/4] workqueue: Unbind kworkers before sending them to exit()

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Date:   Thu, 1 Dec 2022 11:01:47 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        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 v6 4/4] workqueue: Unbind kworkers before sending them to exit()

On Tue, Nov 29, 2022 at 2:31 AM Valentin Schneider <vschneid@...hat.com> wrote:

> @@ -3627,8 +3668,11 @@ static bool wq_manager_inactive(struct worker_pool *pool)
>  static void put_unbound_pool(struct worker_pool *pool)
>  {
>         DECLARE_COMPLETION_ONSTACK(detach_completion);
> +       struct list_head cull_list;
>         struct worker *worker;
>
> +       INIT_LIST_HEAD(&cull_list);
> +
>         lockdep_assert_held(&wq_pool_mutex);
>
>         if (--pool->refcnt)
> @@ -3651,17 +3695,19 @@ 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.
>          */
> +       mutex_lock(&wq_pool_attach_mutex);
>         rcuwait_wait_event(&manager_wait, wq_manager_inactive(pool),
>                            TASK_UNINTERRUPTIBLE);
>         pool->flags |= POOL_MANAGER_ACTIVE;

Hello, Valentin

I'm afraid it might deadlock here.

If put_unbound_pool() is called while manage_workers() is sleeping
on allocating memory, put_unbound_pool() will get the wq_pool_attach_mutex
earlier than the manager which prevents the manager from getting the
lock to attach the newly created worker and deadlock.

I think mutex_lock(&wq_pool_attach_mutex) can be moved into
wq_manager_inactive(), and handle it in the same way as pool->lock.

>
>         while ((worker = first_idle_worker(pool)))
> -               destroy_worker(worker);
> +               set_worker_dying(worker, &cull_list);
>         WARN_ON(pool->nr_workers || pool->nr_idle);
>         raw_spin_unlock_irq(&pool->lock);
>
> -       mutex_lock(&wq_pool_attach_mutex);
> -       if (!list_empty(&pool->workers))
> +       wake_dying_workers(&cull_list);
> +
> +       if (!list_empty(&pool->workers) || !list_empty(&pool->dying_workers))
>                 pool->detach_completion = &detach_completion;
>         mutex_unlock(&wq_pool_attach_mutex);
>
> --
> 2.31.1
>