linux-kernel - Re: [RFC PATCH 05/13] list.h: Fix parentheses around macro pointer parameter use

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Message-ID: <ZFjoJqbDn/BL1GQT@smile.fi.intel.com>
Date:   Mon, 8 May 2023 15:16:38 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Howells <dhowells@...hat.com>,
        Ricardo Martinez <ricardo.martinez@...ux.intel.com>
Subject: Re: [RFC PATCH 05/13] list.h: Fix parentheses around macro pointer
 parameter use

On Thu, May 04, 2023 at 04:05:19PM -0400, Mathieu Desnoyers wrote:
> Add missing parentheses around use of macro argument "pos" in those
> patterns to ensure operator precedence behaves as expected:
> 
> - typeof(*pos)
> - pos->member
> - "x = y" is changed for "x = (y)", because "y" can be an expression
>   containing a comma if it is the result of the expansion of a macro such
>   as #define eval(...) __VA_ARGS__, which would cause unexpected operator
>   precedence. This use-case is far-fetched, but we have to choose one
>   way or the other (with or without parentheses) for consistency,
> - x && y is changed for (x) && (y).
> 
> Remove useless parentheses around use of macro parameter (head) in the
> following pattern:
> 
> - list_is_head(pos, (head))
> 
> Because comma is the lowest priority operator already, so the extra pair
> of parentheses is redundant.

But strictly speaking it might be something like

	list_...(..., (a, b))

where (a, b) is the head. No?

> This corrects the following usage pattern where operator precedence is
> unexpected:
> 
>   LIST_HEAD(testlist);
> 
>   struct test {
>           struct list_head node;
>           int a;
>   };
> 
>   // pos->member issue
>   void f(void)
>   {
>           struct test *t1;
>           struct test **t2 = &t1;
> 
>           list_for_each_entry((*t2), &testlist, node) {   /* works */
>                   //...
>           }
>           list_for_each_entry(*t2, &testlist, node) {     /* broken */
>                   //...

Me still in doubt. But it's up to maintainers.

>           }
>   }
> 
>   // typeof(*pos) issue
>   void f2(void)
>   {
>           struct test *t1 = NULL, *t2;
> 
>           t2 = list_prepare_entry((0 + t1), &testlist, node);     /* works */
>           t2 = list_prepare_entry(0 + t1, &testlist, node);       /* broken */
>   }
> 
> Note that the macros in which "pos" is also used as an lvalue probably
> don't suffer from the lack of parentheses around "pos" in typeof(*pos),
> but add those nevertheless to keep everything consistent.

-- 
With Best Regards,
Andy Shevchenko