Re: [PATCH 1/1 V2] workqueue: remove the del_timer_sync()s in maybe_create_worker()

From: Lai Jiangshan
Date: Tue Jul 15 2014 - 21:23:04 EST

On 07/15/2014 05:42 AM, Thomas Gleixner wrote:
> On Tue, 15 Jul 2014, Lai Jiangshan wrote:
>> 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).
> No. The del_timer_sync() code holds the base lock with interrupts
> disabled. So it can't be interrupted.

The del_timer_sync() waits via cpu_relax() without interrupts
disabled. Could you wipe out my concern?

> .

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