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

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Date:   Fri, 5 May 2023 13:22:40 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        linux-block@...r.kernel.org
Subject: Re: [RFC PATCH 12/13] blk-mq.h: Fix parentheses around macro
 parameter use

On Fri, May 5, 2023 at 1:08 PM Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> The reason why I think the lvalue of a "=" operator can be argued to be
> "special" is because it is simply invalid to apply many of the C
> operators to an lvalue (e.g. +, -, /, ...),

Mathieu, you are simply objectively wrong.

See here:

  #define m1(x) (x = 2)
  #define m2(x) ((x) = 2)

and then try using the argument "a = b" to those macros.

Guess which one flags it as an error ("lvalue required") and which one does not?

m2 is the only "good" one. Yes, m1 works in 99% of all cases in
practice, but if you want a safer macro, you *will* add the
parentheses.

So *STOP*ARGUING* based on an incorrect "lowest precedence" basis.
Even for the "lowest precedence" case, you have the *same* precedence.

The fact is, assignment is not in any way special operation in macros,
and does not deserve - and should absolutely not have - any special
"doesn't need parentheses around argument" rules.

           Linus