linux-kernel - Re: [PATCH 01 of 12] Core of mmu notifiers

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Date:	Tue, 22 Apr 2008 17:37:38 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andrea Arcangeli <andrea@...ranet.com>
Cc:	Christoph Lameter <clameter@....com>,
	Nick Piggin <npiggin@...e.de>, Jack Steiner <steiner@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	kvm-devel@...ts.sourceforge.net,
	Kanoj Sarcar <kanojsarcar@...oo.com>,
	Roland Dreier <rdreier@...co.com>,
	Steve Wise <swise@...ngridcomputing.com>,
	linux-kernel@...r.kernel.org, Avi Kivity <avi@...ranet.com>,
	linux-mm@...ck.org, Robin Holt <holt@....com>,
	general@...ts.openfabrics.org, Hugh Dickins <hugh@...itas.com>,
	akpm@...ux-foundation.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 01 of 12] Core of mmu notifiers

Andrea Arcangeli a écrit :
> On Tue, Apr 22, 2008 at 04:56:10PM +0200, Eric Dumazet wrote:
>   
>> Andrea Arcangeli a écrit :
>>     
>>> +
>>> +static int mm_lock_cmp(const void *a, const void *b)
>>> +{
>>> +	cond_resched();
>>> +	if ((unsigned long)*(spinlock_t **)a <
>>> +	    (unsigned long)*(spinlock_t **)b)
>>> +		return -1;
>>> +	else if (a == b)
>>> +		return 0;
>>> +	else
>>> +		return 1;
>>> +}
>>> +
>>>       
>> This compare function looks unusual...
>> It should work, but sort() could be faster if the
>> if (a == b) test had a chance to be true eventually...
>>     
>
> Hmm, are you saying my mm_lock_cmp won't return 0 if a==b?
>   
I am saying your intent was probably to test

else if ((unsigned long)*(spinlock_t **)a ==
	    (unsigned long)*(spinlock_t **)b)
		return 0;


Because a and b are pointers to the data you want to compare. You need 
to dereference them.


>> static int mm_lock_cmp(const void *a, const void *b)
>> {
>> 	unsigned long la = (unsigned long)*(spinlock_t **)a;
>> 	unsigned long lb = (unsigned long)*(spinlock_t **)b;
>>
>> 	cond_resched();
>> 	if (la < lb)
>> 		return -1;
>> 	if (la > lb)
>> 		return 1;
>> 	return 0;
>> }
>>     
>
> If your intent is to use the assumption that there are going to be few
> equal entries, you should have used likely(la > lb) to signal it's
> rarely going to return zero or gcc is likely free to do whatever it
> wants with the above. Overall that function is such a slow path that
> this is going to be lost in the noise. My suggestion would be to defer
> microoptimizations like this after 1/12 will be applied to mainline.
>
> Thanks!
>
>   
Hum, it's not a micro-optimization, but a bug fix. :)

Sorry if it was not clear




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