linux-kernel - Re: [GIT PULL] locking changes for v6.13

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Message-ID: <CAHk-=wi8C2yZF_y_T180-v+dSZAhps5QghS_2tKfn-+xAghYPQ@mail.gmail.com>
Date: Tue, 19 Nov 2024 12:56:31 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, 
	Boqun Feng <boqun.feng@...il.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: [GIT PULL] locking changes for v6.13

On Mon, 18 Nov 2024 at 01:03, Ingo Molnar <mingo@...nel.org> wrote:
>
>  - <linux/cleanup.h>:
>     - Add if_not_cond_guard() conditional guard helper (David Lechner)

I've pulled this, but I'm unhappy.

This macro generates actively wrong code if it happens to be inside an
if-statement or a loop without a block.

IOW, code like this:

    for (iterate-over-something)
        if_not_guard(a)
            return -BUSY;

looks like will build fine, but will generate completely incorrect code.

Honestly, just switching the order of the BUILD_BUG_ON() and the
CLASS() declaration looks like it would have fixed this (because then
the '_id' won't be in scope of the subsequent if-statement any more),
but I'm unhappy with how apparently nobody even bothered to think
about such a fundamental issue with macros.

Macros that expand to statements absolutely *ALWAYS* need to deal with
"what if we're in a single-statement situation?"

              Linus