linux-kernel - Re: [PATCH v3 5/5] arm64: mte: add compression support to mteswap.c

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Date:   Tue, 18 Jul 2023 17:13:02 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     catalin.marinas@....com, will@...nel.org, pcc@...gle.com,
        andreyknvl@...il.com, linux@...musvillemoes.dk,
        yury.norov@...il.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, eugenis@...gle.com,
        syednwaris@...il.com, william.gray@...aro.org
Subject: Re: [PATCH v3 5/5] arm64: mte: add compression support to mteswap.c

On Tue, Jul 18, 2023 at 12:48:00PM +0200, Alexander Potapenko wrote:

...

> > > +void _mte_free_saved_tags(void *storage)
> > > +{
> > > +     unsigned long handle = xa_to_value(storage);
> > > +     int size;
> > > +
> > > +     if (!handle)
> > > +             return;
> >
> > Perhaps
> >
> >         unsigned long handle;
> >
> >         handle = xa_to_value(storage);
> >         if (!handle)
> >                 return;
> 
> I don't have a strong preference and am happy to change this, but, out
> of curiosity, why do you think it is better?
> This pattern (calling (even non-)trivial functions when declaring
> variables) is widely used across the kernel.
> Or is it just for consistency with how `handle` is used in the rest of the file?

Ah, it's pure maintenance and error prone approach in case some code is sneezed
in between.

Imagine that you have


	foo = bar(x);
	...many lines that by some reason don't make one page on the screen...
	if (!foo)
		...do something...

Now if by unsuccessful rebase or by non-experienced developer we got

	foo = bar(x);
	...part 1 of many lines that by some reason don't make one page on the screen...
	baz(foo);
	...part 2 of many lines that by some reason don't make one page on the screen...
	if (!foo)
		...do something...

the compiler will eliminate the check — you got your mine on the nice minefield!

> > > +}

-- 
With Best Regards,
Andy Shevchenko