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

From: Peter Zijlstra <peterz@infradead.org>
To: Arnd Bergmann <arnd@kernel.org>
Cc: linux-kernel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Jens Axboe <axboe@kernel.dk>, Jian Cai <jiancai@google.com>,
	Guenter Roeck <linux@roeck-us.net>,
	"Huang, Ying" <ying.huang@intel.com>,
	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@googlegroups.com
Subject: Re: [PATCH] [v2] smp: fix smp_call_function_single_async prototype
Date: Thu, 6 May 2021 12:10:52 +0200	[thread overview]
Message-ID: <YJPArKvd+GuiBwRk@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210505211300.3174456-1-arnd@kernel.org>

On Wed, May 05, 2021 at 11:12:42PM +0200, Arnd Bergmann wrote:
> 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);
> 
> It does seem that the usage of the call_single_data without cache line
> alignment should still be allowed by the smp code, so just change the
> function prototype so it accepts both, but leave the default alignment
> unchanged for the other users. This seems better to me than adding
> a local hack to shut up an otherwise correct warning in the caller.
> 
> Link: https://lore.kernel.org/linux-block/20210330230249.709221-1-jiancai@google.com/
> Link: https://github.com/ClangBuiltLinux/linux/issues/1328
> Acked-by: Jens Axboe <axboe@kernel.dk>
> Cc: Jian Cai <jiancai@google.com>
> Cc: Guenter Roeck <linux@roeck-us.net>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: "Huang, Ying" <ying.huang@intel.com>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Eric Dumazet <eric.dumazet@gmail.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Thanks!