linux-kernel - RE: [RFC PATCH 12/13] blk-mq.h: Fix parentheses around macro parameter use

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Message-ID: <71d4279ca9524b2382059cd2f723e20e@AcuMS.aculab.com>
Date:   Sat, 6 May 2023 15:45:54 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Subject: RE: [RFC PATCH 12/13] blk-mq.h: Fix parentheses around macro
 parameter use

From: Linus Torvalds
> Sent: 05 May 2023 20:55
....
> > The other exception I would add is when a parameter is used as an
> > lvalue, as:
> >
> > #define m(x) do { x = 2; } while (0)
> 
> I really don't understand why you think '=' is so special. It's very
> much not special.
> 
> It happens to have the lowest precedence, sure, but the keyword is "happens".

And consider what happens if you try:
	m(a ? b : c)

Personally I'd avoid using parameters as lvalues if at all possible.
It is much better to have:
	#define m(x) do { *(x) = 2; } while (0)
and require the caller do m(&foo) to make it obvious the value is changed.
(Apart from loop definitions...)

Things like the C++ 'int &arg' make it hard to scan read/search
code for places where variables get changed.

	David

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