Re: [PATCH] [v2] smp: fix smp_call_function_single_async prototype

From: Arnd Bergmann <arnd@kernel.org>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Jens Axboe <axboe@kernel.dk>, Jian Cai <jiancai@google.com>,
	Guenter Roeck <linux@roeck-us.net>,
	Peter Zijlstra <peterz@infradead.org>,
	Borislav Petkov <bp@suse.de>,
	Eric Dumazet <eric.dumazet@gmail.com>,
	Juergen Gross <jgross@suse.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Thomas Gleixner <tglx@linutronix.de>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Ingo Molnar <mingo@kernel.org>,
	Frederic Weisbecker <frederic@kernel.org>,
	He Ying <heying24@huawei.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH] [v2] smp: fix smp_call_function_single_async prototype
Date: Thu, 6 May 2021 10:30:49 +0200	[thread overview]
Message-ID: <CAK8P3a3kZ9_VoKV+2eZh=WqncRqFKzRmRHUjAT9iFMtJpKzb1w@mail.gmail.com> (raw)
In-Reply-To: <877dkcs2h8.fsf@yhuang6-desk1.ccr.corp.intel.com>

On Thu, May 6, 2021 at 10:14 AM Huang, Ying <ying.huang@intel.com> wrote:
>
> Arnd Bergmann <arnd@kernel.org> writes:
>
> > On Thu, May 6, 2021 at 3:20 AM Huang, Ying <ying.huang@intel.com> wrote:
> >>
> >> Arnd Bergmann <arnd@kernel.org> writes:
> >>
> >> > From: Arnd Bergmann <arnd@arndb.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
>
> We cannot avoid type cast in Linux kernel, such as container_of(), is
> there some difference here?

container_of() does not cause any alignment problems. Assuming the outer
structure is aligned correctly, then the inner structure also is.

> > 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.
>
> In its raw form as above, this looks bad.  If we encapsulate it, it may
> look better, for example,
>
> static inline int __smp_call_function_single_async(int cpu, struct __call_single_data *csd)
> {
>         smp_call_function_single_async(cpu, (call_single_data_t *)csd);
> }
>
> Then, we can do
>
>         __smp_call_function_single_async(cpu, &rq->csd);

Same problem, it's still calling a function that expects stricter alignment.
It would work if we do it the other way around though:

static inline int smp_call_function_single_async(int cpu,
call_single_data_t *csd)
{
        return __smp_call_function_single_async(cpu, csd);
}

That should even work without the cast.

        Arnd