linux-kernel - Re: [PATCH 2/2] workqueue: Keep activate-order equals to queue

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Date:	Wed, 19 Sep 2012 18:13:24 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] workqueue: Keep activate-order equals to queue_work()-order

On 09/19/2012 01:08 AM, Tejun Heo wrote:
> On Tue, Sep 18, 2012 at 10:05:19AM -0700, Tejun Heo wrote:
>> On Tue, Sep 18, 2012 at 04:36:53PM +0800, Lai Jiangshan wrote:
>>> The whole workqueue.c keeps activate-order equals to queue_work()-order
>>> in any given cwq except workqueue_set_max_active().
>>>
>>> If this order is not kept, something may be not good:
>>>
>>> first_work_fn() { release some resource; }
>>> second_work_fn() { wait and request the resource; use resource; }
>>>
>>> 1. user queues the first work.	# ->max_active is low, is queued on ->delayed_works.
>>> 2. someone increases the >max_active via workqueue_set_max_active()
>>> 3. user queues the second work.	# queued on cwq->pool.
>>>
>>> When the second work is launched to execute, it waits the first work
>>> to release the resource. But the first work is still in ->delayed_works,
>>> it waits the first work to finish and them it can be activated.
>>>
>>> It is bad. we fix it by activating the first work in the step 2.
>>>
>>> I can't fully determine that it is workqueue's responsibility
>>> or the user's responsibility.
>>> If it is workqueue's responsibility, the patch needs go to -stable.
>>> If it is user's responsibility. it is a nice cleanup, it can go to for-next.
>>> I prefer it is workqueue's responsibility.
>>
>> Unless max_active == 1, workqueue doesn't give any guarantee on
>> execution order.  I don't think we need to care about this.
> 
> That said, I kinda like the patches.  Can you please update the
> description on the second patch to something along the line of "use
> common set_max_active logic which immediately makes use of the newly
> increased max_mactive if there are delayed work items and also happens
> to keep activation ordering"?
> 
> Thanks.
> 

Updated.
Thanks,
Lai

>From 314d43f087c85b11a29be0555f32deeb742bf18e Mon Sep 17 00:00:00 2001
From: Lai Jiangshan <laijs@...fujitsu.com>
Date: Tue, 18 Sep 2012 16:26:30 +0800
Subject: [PATCH] workqueue: use common cwq_set_max_active() for
 workqueue_set_max_active()

workqueue_set_max_active() may increase ->max_active without activating
delayed works. And it may cause the activation order doesn't equal to
to queue_work()-order.

To make things consist, we use common cwq_set_max_active() logic which
immediately makes use of the newly increased max_mactive if there are
delayed work items and also keep activation ordering.

Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
---
 kernel/workqueue.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index d0ca063..8783414 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3458,7 +3458,7 @@ void workqueue_set_max_active(struct workqueue_struct *wq, int max_active)
 
 		if (!(wq->flags & WQ_FREEZABLE) ||
 		    !(gcwq->flags & GCWQ_FREEZING))
-			get_cwq(gcwq->cpu, wq)->max_active = max_active;
+			cwq_set_max_active(get_cwq(gcwq->cpu, wq), max_active);
 
 		spin_unlock_irq(&gcwq->lock);
 	}
-- 
1.7.4.4
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