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

[Linus Torvalds <torvalds@linux-foundation.org>]

On Mon, 18 Nov 2024 at 01:03, Ingo Molnar <mingo@kernel.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