linux-kernel - Re: [PATCH 02/10] workqueue: destroy_worker() should destroy idle workers only

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Date:	Tue, 6 May 2014 23:58:08 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/10] workqueue: destroy_worker() should destroy idle
 workers only

On Mon, May 5, 2014 at 9:13 PM, Tejun Heo <tj@...nel.org> wrote:
> Hello,
>
> On Sun, Apr 27, 2014 at 12:08:57PM +0800, Lai Jiangshan wrote:
>> @@ -1692,9 +1691,8 @@ static struct worker *alloc_worker(void)
>>   * create_worker - create a new workqueue worker
>>   * @pool: pool the new worker will belong to
>>   *
>> - * Create a new worker which is bound to @pool.  The returned worker
>> - * can be started by calling start_worker() or destroyed using
>> - * destroy_worker().
>> + * Create a new worker which is bound to @pool.
>> + * The new worker should be started and enter idle by start_worker().
>
> Hmm... we used to have a path where a worker is created and then
> destroyed without being started.  IIRC, it was on the CPU online
> failure path.  A worker was created for the CPU coming online and if
> the online operation failed the created worker was shut down without
> being started.  Right, we no longer shutdown per-cpu pools on offline
> so this doesn't matter anymore.  Might worthwhile to note in the patch
> description tho.
>
>> @@ -1815,6 +1812,7 @@ static int create_and_start_worker(struct worker_pool *pool)
>>   * @worker: worker to be destroyed
>>   *
>>   * Destroy @worker and adjust @pool stats accordingly.
>> + * The worker should be idle(WORKER_IDLE).
>
> Just write "The worker should be idle."  Also, in general, can you
> please put a space before opening parenthesis?
>
>>   *
>>   * CONTEXT:
>>   * spin_lock_irq(pool->lock) which is released and regrabbed.
>> @@ -1828,13 +1826,13 @@ static void destroy_worker(struct worker *worker)
>>
>>       /* sanity check frenzy */
>>       if (WARN_ON(worker->current_work) ||
>> -         WARN_ON(!list_empty(&worker->scheduled)))
>> +         WARN_ON(!list_empty(&worker->scheduled)) ||
>> +         WARN_ON(!(worker->flags & WORKER_IDLE)) ||
>> +         WARN_ON(pool->nr_workers == 1 && !list_empty(&pool->worklist)))
>
> I'm not sure about the pool condition check.  It's kinda overreaching
> to check for it from worker destruction.

"pool->nr_workers == 1", it is the last worker of the pool, so I add this check.

I will remove it.

>
>> @@ -3589,6 +3587,7 @@ static void put_unbound_pool(struct worker_pool *pool)
>>       mutex_lock(&pool->manager_mutex);
>>       spin_lock_irq(&pool->lock);
>>
>> +     WARN_ON(pool->nr_workers != pool->nr_idle);
>
> Does this condition detect anything new from the condition below?

It is already ensured that all workers are idle since pool->ref==0
and manager_arb is held.

This additional WARN_ON() does double check.
It tells the reviewers that destroying all workers on idle_list
equals to destroying all workers of the pool.

I can remove this WARN_ON().

>
>>       while ((worker = first_worker(pool)))
>>               destroy_worker(worker);
>>       WARN_ON(pool->nr_workers || pool->nr_idle);
>
> Thanks.
>
> --
> tejun
> --
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