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

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Date:   Thu, 29 Apr 2021 09:54:10 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Arnd Bergmann <arnd@...nel.org>, linux-kernel@...r.kernel.org
Cc:     Arnd Bergmann <arnd@...db.de>, Jian Cai <jiancai@...gle.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Peter Zijlstra <peterz@...radead.org>,
        "Huang, Ying" <ying.huang@...el.com>, 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>
Subject: Re: [PATCH] smp: fix smp_call_function_single_async prototype

On 4/29/21 9:09 AM, Arnd Bergmann wrote:
> 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);
> 
> 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.

I think that's the right approach, rather than work-around it in eg
blk-mq.

Acked-by: Jens Axboe <axboe@...nel.dk>

-- 
Jens Axboe