Re: [PATCH 01 of 12] Core of mmu notifiers

[Eric Dumazet]

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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