linux-kernel - Re: [PATCH 27/35] workqueue: implement concurrency managed dynamic worker pool

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Date:	Fri, 9 Jul 2010 17:11:38 +0800
From:	Yong Zhang <yong.zhang@...driver.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	torvalds@...ux-foundation.org, mingo@...e.hu,
	linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, rusty@...tcorp.com.au,
	cl@...ux-foundation.org, dhowells@...hat.com,
	arjan@...ux.intel.com, oleg@...hat.com, axboe@...nel.dk,
	fweisbec@...il.com, dwalker@...eaurora.org,
	stefanr@...6.in-berlin.de, florian@...kler.org,
	andi@...stfloor.org, mst@...hat.com, randy.dunlap@...cle.com
Subject: Re: [PATCH 27/35] workqueue: implement concurrency managed dynamic
 worker pool

On Mon, Jun 28, 2010 at 11:04:15PM +0200, Tejun Heo wrote:
> +static bool maybe_create_worker(struct global_cwq *gcwq)
> +{
> +	if (!need_to_create_worker(gcwq))
> +		return false;
> +restart:
> +	/* if we don't make progress in MAYDAY_INITIAL_TIMEOUT, call for help */
> +	mod_timer(&gcwq->mayday_timer, jiffies + MAYDAY_INITIAL_TIMEOUT);
> +
> +	while (true) {
> +		struct worker *worker;
> +
> +		spin_unlock_irq(&gcwq->lock);
> +
> +		worker = create_worker(gcwq, true);
> +		if (worker) {
> +			del_timer_sync(&gcwq->mayday_timer);
> +			spin_lock_irq(&gcwq->lock);
> +			start_worker(worker);
> +			BUG_ON(need_to_create_worker(gcwq));
> +			return true;
> +		}
> +
> +		if (!need_to_create_worker(gcwq))
> +			break;
> +
> +		spin_unlock_irq(&gcwq->lock);
> +		__set_current_state(TASK_INTERRUPTIBLE);
> +		schedule_timeout(CREATE_COOLDOWN);
> +		spin_lock_irq(&gcwq->lock);
> +		if (!need_to_create_worker(gcwq))
> +			break;
> +	}
> +
> +	spin_unlock_irq(&gcwq->lock);

A little worried about the lock operation. We may call spin_unlock_irq() twice
under some special situation. Couldn't that happen? Or Am I missing something?

And a rough patch for this issue if needed:
---
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 2eb9fbd..84a9cb9 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1427,10 +1427,11 @@ restart:
 			return true;
 		}
 
-		if (!need_to_create_worker(gcwq))
+		if (!need_to_create_worker(gcwq)) {
+			spin_lock_irq(&gcwq->lock);
 			break;
+		}
 
-		spin_unlock_irq(&gcwq->lock);
 		__set_current_state(TASK_INTERRUPTIBLE);
 		schedule_timeout(CREATE_COOLDOWN);
 		spin_lock_irq(&gcwq->lock);

> +	del_timer_sync(&gcwq->mayday_timer);
> +	spin_lock_irq(&gcwq->lock);
> +	if (need_to_create_worker(gcwq))
> +		goto restart;
> +	return true;
> +}
> +
--
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