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

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Date:   Tue, 6 Nov 2018 09:20:09 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     bvanassche@....org
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 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.
>
> Bart.

Actually the defined macro is used multiple spots in if statements
throughout the kernel.

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'.

Thanks.

- Alex