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 16:08:07 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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 2023-05-05 15:54, Linus Torvalds wrote:
> On Fri, May 5, 2023 at 11:49 AM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:

[...]

>> 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".
> 
> I think you are confused by the non-C languages that make assignment
> be not an expression operator, but a statement.

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. +, -, /, ...), which leads me to think that 
there are no valid lvalue parameters which can cause unexpected operator 
precedence.

That being said, just having to *think* about it is wasted brain power, 
so I am in favor of just adding the parentheses for lvalues as well.

> So I think you are technically correct in that the parentheses aren't
> _needed_, but the above is still the same case that in many other
> situations parentheses aren't technically *needed*, but not having to
> think about it is better than having to do so.

Yes, so no exception for the lvalue of an assignment, therefore giving:

#define m(x) do { (x) = 2; } while (0)

If we are OK with this, I will go ahead and update my patch set accordingly.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com