Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.34 as permitted sender) client-ip=23.128.96.34;
MIME-Version: 1.0
References: <20231203221151.794615-1-ubizjak@gmail.com> <CAHk-=wiS6nyWNjaTW_XL1ec3+-=tOszj+_sWGfPv9RG5WX5isQ@mail.gmail.com>
In-Reply-To: <CAHk-=wiS6nyWNjaTW_XL1ec3+-=tOszj+_sWGfPv9RG5WX5isQ@mail.gmail.com>
From:   Uros Bizjak <ubizjak@gmail.com>
Date:   Sun, 3 Dec 2023 23:49:51 +0100
Message-ID: <CAFULd4amFXV1WB0O64BihiLCtmZw5B65Rf=7dTtni5fnvBoPWQ@mail.gmail.com>
Subject: Re: [PATCH -tip 1/3] x86/percpu: Fix "const_pcpu_hot" version
 generation failure
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Ingo Molnar <mingo@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Sun, Dec 3, 2023 at 11:19=E2=80=AFPM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Mon, 4 Dec 2023 at 07:12, Uros Bizjak <ubizjak@gmail.com> wrote:
> >
> > +/*
> > + * The generic per-cpu infrastrucutre is not suitable for
> > + * reading const-qualified variables.
> > + */
> > +#define this_cpu_read_const(pcp)       ({ BUG(); (typeof(pcp))0; })
>
> NAK. Absolutely not.
>
> No way in hell is it acceptable to make this a run-time BUG. If it
> doesn't work, it needs to be a compile failure. End of story.

Thanks - BUILD_BUG() also works here and is indeed more appropriate:

/**
 * BUILD_BUG - break compile if used.
 *
 * If you have some code that you expect the compiler to eliminate at
 * build time, you should use BUILD_BUG to detect if it is
 * unexpectedly used.
 */

v2 is in testing.

Thanks,
Uros.