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

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Message-ID: <CAHk-=wjfgCa-u8h9z+8U7gaKK6PnRCpws1Md9wYSSXywUxoUSA@mail.gmail.com>
Date:   Thu, 4 May 2023 10:16:36 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH 4/4] llist.h: Fix parentheses around macro pointer
 parameter use

On Thu, May 4, 2023 at 7:54 AM Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
> +#define list_prepare_entry(pos, head, member) \
> +       ((pos) ? : list_entry(head, typeof(*pos), member))
>
> So even though the fact that "pos" is used as an lvalue specifically in
> llist_for_each_entry_safe() makes it so that the parentheses are not
> strictly required around "pos" in typeof(*pos), I argue that we should
> still add those for consistency.

Ack. It may not matter in reality because of how 'pos' is supposed to
be just a local variable name, but I agree that for consistency our
macros should still follow the usual pattern.

Of course, *because* of how 'pos' is not some random expression, and
is supposed to be that local variable, and because of how it is used,
it must always violate the whole "only use once" usual pattern,.

Exactly the same way the member name is also typically used multiple
times because of how it's not an expression, but really a "part of the
syntax".

So an alternative might be that we should use a different syntax for
those things and make it clear that they are not normal expressions.
Some people use upper-case names for special things like that to make
them stand out as "not normal" (kind of the same way upper-case macros
themselves were a warning that they weren't normal and might evaluate
arguments multiple times).

                   Linus