linux-kernel - Re: [PATCH] [v2] smp: fix smp_call_function_single

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Message-ID: <CAK8P3a1SBDXqHE5FgG_WfzrcbeT6V6kg5T+xTGU8Cp_vLLdMqA@mail.gmail.com>
Date:   Thu, 6 May 2021 09:54:55 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>, Jian Cai <jiancai@...gle.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...e.de>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Juergen Gross <jgross@...e.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        He Ying <heying24@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] [v2] smp: fix smp_call_function_single_async prototype

On Thu, May 6, 2021 at 3:20 AM Huang, Ying <ying.huang@...el.com> wrote:
>
> Arnd Bergmann <arnd@...nel.org> writes:
>
> > From: Arnd Bergmann <arnd@...db.de>
> >
> > As of commit 966a967116e6 ("smp: Avoid using two cache lines for struct
> > call_single_data"), the smp code prefers 32-byte aligned call_single_data
> > objects for performance reasons, but the block layer includes an instance
> > of this structure in the main 'struct request' that is more senstive
> > to size than to performance here, see 4ccafe032005 ("block: unalign
> > call_single_data in struct request").
> >
> > The result is a violation of the calling conventions that clang correctly
> > points out:
> >
> > block/blk-mq.c:630:39: warning: passing 8-byte aligned argument to 32-byte aligned parameter 2 of 'smp_call_function_single_async' may result in an unaligned pointer access [-Walign-mismatch]
> >                 smp_call_function_single_async(cpu, &rq->csd);
>
> Can this be silenced by
>
>                 smp_call_function_single_async(cpu, (call_single_data_t *)&rq->csd);

Probably, but casting from smaller alignment to larger alignment is undefined
behavior and I'd rather not go there in case this triggers some runtime
misbehavior or ubsan check in the future. Making the function accept a
pointer with the smaller alignment avoids getting into undefined behavior
and doesn't require a cast.

       Arnd