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

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Message-ID: <1541526521.196084.184.camel@acm.org>
Date:   Tue, 06 Nov 2018 09:48:41 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     linux@...musvillemoes.dk,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Christoph Lameter <cl@...ux.com>, guro@...com,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] slab.h: Avoid using & for logical and of booleans

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@....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.