netdev - Re: [PATCH] net: remove redundant checking for sock timer state

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Date:	Fri, 1 Feb 2013 15:25:46 +0800
From:	Ying Xue <ying.xue@...driver.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Xue Ying <ying.xue0@...il.com>, David Miller <davem@...emloft.net>,
	<netdev@...r.kernel.org>
Subject: Re: [PATCH] net: remove redundant checking for sock timer state

Eric Dumazet wrote:
> On Fri, 2013-02-01 at 15:14 +0800, Xue Ying wrote:
>> Eric Dumazet wrote:
> 
>>> I had the same reaction but maybe its not anymore a valid thing.
>>>
>>> Before commit 55c888d6d ([PATCH] timers fixes/improvements) there was
>>> indeed a significant cost calling del_timer() because of unconditional
>>> spinlock acquisition.
>>>
>>> But nowadays del_timer() doesn't blindly lock the spinlock.
>>>
>>> So I guess we could change all occurrences of :
>>>
>>> if (timer_pending(X))
>>>     del_timer(X);
>>>
>>> It would save some bytes of code.
>>>   
>> Eric, thanks for your explanation and suggestion.
>>
>> But I cannot understand why we should first call timer_pending() before 
>> del_timer() in your proposal.
>> By my understanding, we might get an unreal timer pending state out of 
>> timer base lock (ie, lock_timer_base()),
>> and the "unreal" is only for pending state, on the contrary, the value 
>> is real for inactive sate.
>> So calling timer_pending() out of timer base lock scope can make us 
>> avoid some unnecessary grabbing spin lock operations.
>> However, in del_timer() there already has placed a timer_pending() 
>> before lock_timer_base() is called. So why do we need
>> another before calling del_timer()?
>>
>> Please you explain more.
> 
> I think you misunderstood me.
> 
> I said that the old construct :
> 
> if (timer_pending(X))
>     del_timer(X);
> 
> could be changed to
> 
> del_timer(X);
> 
> Thats what your patch does.
> 
> But instead of changing sk_stop_timer(), I suggested you do a patch on
> whole net tree.
> 

Fine :) I will do.

Regards,
Ying

> 
> 
> 

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