linux-kernel - Re: [PATCH 1/1 V2] workqueue: remove the del_timer_sync()s in maybe_create

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Date:	Tue, 15 Jul 2014 05:18:13 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Tejun Heo <tj@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1 V2] workqueue: remove the del_timer_sync()s in maybe_create_worker()

On 07/14/2014 11:33 PM, Thomas Gleixner wrote:
> On Mon, 14 Jul 2014, Tejun Heo wrote:
> 
>> Hello,
>>
>> On Mon, Jul 14, 2014 at 04:13:21PM +0800, Lai Jiangshan wrote:
>>> It is said in the document that the timer which is being
>>> deleted by del_timer_sync() should not be restarted:
>>>   Synchronization rules: Callers must prevent restarting of
>>>   the timer, otherwise this function is meaningless.
>>>
>>> Repeating timer may cause the del_timer_sync() spin longer,
>>> or even spin forever in very very very very extreme condition.
>>
>> I'm fairly sure del_timer_sync() can delete self-requeueing timers.
>> The implementation busy-waits if the queued timer is the currently
>> executing one and dequeues only while the timer isn't running which
>> should be able to handle self-requeueing ones just fine.  Thomas,
>> del_timer_sync() can reliably delete self-requeueing ones, right?
> 
> Yes. 

The comments of the del_timer_sync() needs to be updated
if I did not misunderstood?

> If the timer callback is running on the other cpu, then it waits
> for the callback to finish before checking whether the timer is
> enqueued or not.

The syncer may be interrupted here, after it comes back, the timer
may be running again (and maybe again and again).

Current code is an acceptable ostrich algorithm, but the documents
needs to be updated. Or re-implement it as the way as cancel_work_sync()
which has similar but stronger semantics, it disables requeueing when
syncing.



> 
> Thanks,
> 
> 	tglx
> 
> 
> .
> 

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