Re: [RFC] resource: Avoid unnecessary resource tree walking in __region_intersects()

[Huang, Ying]

Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> writes:

> On Thu, Oct 24, 2024 at 08:30:39PM +0800, Huang, Ying wrote:
>> Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> writes:
>> > On Wed, Oct 23, 2024 at 02:07:52PM -0700, Dan Williams wrote:
>> >> Andy Shevchenko wrote:
>> >> > On Fri, Oct 11, 2024 at 09:06:37AM +0800, Huang, Ying wrote:
>> >> > > David Hildenbrand <david@xxxxxxxxxx> writes:
>> >> > > > On 10.10.24 08:55, Huang Ying wrote:
>
> ...
>
>> >> > > > 	for ((_p) = (_root)->child; (_p); (_p) = next_resource_XXX(_root, _p))
>> >> > > 
>> >> > > Yes.  This can improve code readability.
>> >> > > 
>> >> > > A possible issue is that "_root" will be evaluated twice in above macro
>> >> > > definition.  IMO, this should be avoided.
>> >> > 
>> >> > Ideally, yes. But how many for_each type of macros you see that really try hard
>> >> > to achieve that? I believe we shouldn't worry right now about this and rely on
>> >> > the fact that root is the given variable. Or do you have an example of what you
>> >> > suggested in the other reply, i.e. where it's an evaluation of the heavy call?
>> >> > 
>> >> > > Do you have some idea about
>> >> > > how to do that?  Something like below?
>> >> > > 
>> >> > > #define for_each_resource_XXX(_root, _p)                                \
>> >> > > 	for (typeof(_root) __root = (_root), __p = (_p) = (__root)->child; \
>> >> > > 	     __p && (_p); (_p) = next_resource_XXX(__root, _p))
>> >> > 
>> >> > This is a bit ugly :-( I would avoid ugliness as long as we have no problem to
>> >> > solve (see above).
>> >> 
>> >> Using a local defined variable to avoid double evaluation is standard
>> >> practice. I do not understand "avoid ugliness as long as we have no problem to
>> >> solve", the problem to solve will be if someone accidentally does
>> >> something like "for_each_resource_descendant(root++, res)". *That* will
>> >> be a problem when someone finally realizes that the macro is hiding a
>> >> double evaluation.
>> >
>> > Can you explain, why do we need __p and how can we get rid of that?
>> > I understand the part of the local variable for root.
>> 
>> If don't use '__p', the macro becomes
>> 
>> #define for_each_resource_XXX(_root, _p)                                \
>> 	for (typeof(_root) __root = (_root), (_p) = (__root)->child; \
>> 	     (_p); (_p) = next_resource_XXX(__root, _p))
>> 
>> Where, '_p' must be a variable name, and it will be a new variable
>> inside for loop and mask the variable with same name outside of macro.
>> IIUC, this breaks the macro convention in kernel and has subtle variable
>> masking semantics.
>
> Yep.
>
> In property.h nobody cares about evaluation which makes the macro as simple as
>
> #define for_each_resource_XXX(_root, _p)		\
> 	for (_p = next_resource_XXX(__root, NULL); _p;	\
> 	     _p = next_resource_XXX(__root, _p))
>
> (Dan,
>  that's what I called to avoid solving issues we don't have and most likely
>  will never have.)
>
> but if you want to stick with your variant some improvements can be done:

I still prefer to solve possible issues if the solution isn't too
complex.

> #define for_each_resource_XXX(_root, _p)				\
> 	for (typeof(_root) __root = (_root), __p = _p = __root->child;	\
> 	     __p && _p; _p = next_resource_XXX(__root, _p))
>
>
> 1) no need to have local variable in parentheses;
> 2) no need to have iterator in parentheses, otherwise it would be crazy code
> that has put something really wrong there and still expect the thing to work.

Thanks!  You are right.  Will use this in the future versions.

>> >> So no, this proposal is not "ugly", it is a best practice. See the
>> >> definition of min_not_zero() for example.
>> >
>> > I know that there are a lot of macros that look uglier that this one.

--
Best Regards,
Huang, Ying