linux-kernel - Re: [RFC PATCH 2/4] list.h: Fix parentheses around macro pointer parameter use

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Message-ID: <ZFPEEVvHTw2uukun@smile.fi.intel.com>
Date:   Thu, 4 May 2023 17:41:21 +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 2/4] list.h: Fix parentheses around macro pointer
 parameter use

On Wed, May 03, 2023 at 09:29:12PM -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
> 
> 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.
> 
> 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;

I'm not against the patch, but I'm in doubt, looking into this example, it's useful.
Any real use case like above in the Linux kernel, please?

>           list_for_each_entry((*t2), &testlist, node) {   /* works */
>                   //...
>           }
>           list_for_each_entry(*t2, &testlist, node) {     /* broken */
>                   //...
>           }
>   }

-- 
With Best Regards,
Andy Shevchenko