linux-kernel - Re: -Walign-mismatch in block/blk-mq.c

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Message-ID: <99cf90ea-81c0-e110-4815-dd1f7df36cb4@kernel.dk>
Date:   Wed, 10 Mar 2021 13:21:52 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: -Walign-mismatch in block/blk-mq.c

On 3/10/21 11:23 AM, Nathan Chancellor wrote:
> Hi Jens,
> 
> There is a new clang warning added in the development branch,
> -Walign-mismatch, which shows an instance in block/blk-mq.c:
> 
> 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);
>                                                     ^
> 1 warning generated.
> 
> There appears to be some history here as I can see that this member was
> purposefully unaligned in commit 4ccafe032005 ("block: unalign
> call_single_data in struct request"). However, I later see a change in
> commit 7c3fb70f0341 ("block: rearrange a few request fields for better
> cache layout") that seems somewhat related. Is it possible to get back
> the alignment by rearranging the structure again? This seems to be the
> only solution for the warning aside from just outright disabling it,
> which would be a shame since it seems like it could be useful for
> architectures that cannot handle unaligned accesses well, unless I am
> missing something obvious :)

It should not be hard to ensure that alignment without re-introducing
the bloat. Is there some background on why 32-byte alignment is
required?

-- 
Jens Axboe