Re: [PATCH] slab.h: Avoid using & for logical and of booleans

[Bart Van Assche <bvanassche@acm.org>]

On Tue, 2018-11-06 at 09:20 -0800, Alexander Duyck wrote:
> On Mon, Nov 5, 2018 at 4:32 PM Bart Van Assche <bvanassche@acm.org> wrote:
> > 
> > On Mon, 2018-11-05 at 16:11 -0800, Alexander Duyck wrote:
> > > If we really don't care then why even bother with the switch statement
> > > anyway? It seems like you could just do one ternary operator and be
> > > done with it. Basically all you need is:
> > > return (defined(CONFIG_ZONE_DMA) && (flags & __GFP_DMA)) ? KMALLOC_DMA :
> > >         (flags & __GFP_RECLAIMABLE) ? KMALLOC_RECLAIM : 0;
> > > 
> > > Why bother with all the extra complexity of the switch statement?
> > 
> > I don't think that defined() can be used in a C expression. Hence the
> > IS_ENABLED() macro. If you fix that, leave out four superfluous parentheses,
> > test your patch, post that patch and cc me then I will add my Reviewed-by.
> 
> Actually the defined macro is used multiple spots in if statements
> throughout the kernel.

The only 'if (defined(' matches I found in the kernel tree that are not
preprocessor statements occur in Perl code. Maybe I overlooked something?

> The reason for IS_ENABLED is to address the fact that we can be
> dealing with macros that indicate if they are built in or a module
> since those end up being two different defines depending on if you
> select 'y' or 'm'.

>From Documentation/process/coding-style.rst:

Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
symbol into a C boolean expression, and use it in a normal C conditional:

.. code-block:: c

	if (IS_ENABLED(CONFIG_SOMETHING)) {
		...
	}

Bart.