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Sender: Guenter Roeck <groeck7@gmail.com>
Subject: Re: [PATCH] block: Disable -Walign-mismatch for blk-mq.c
To:     Nathan Chancellor <nathan@kernel.org>, Jens Axboe <axboe@kernel.dk>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Jian Cai <jiancai@google.com>,
        Christopher Di Bella <cjdb@google.com>,
        Manoj Gupta <manojgupta@google.com>,
        Luis Lozano <llozano@google.com>, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com
References: <20210408181210.u7cudatr7zcmbmb2@archlinux-ax161>
 <20210408194458.501617-1-nathan@kernel.org>
From:   Guenter Roeck <linux@roeck-us.net>
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Message-ID: <e48bdeee-c8d4-7048-5899-0aa3fe7243ac@roeck-us.net>
Date:   Thu, 8 Apr 2021 12:52:45 -0700
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Precedence: bulk

On 4/8/21 12:44 PM, Nathan Chancellor wrote:
> LLVM 13 adds a new warning, -Walign-mismatch, which has an instance in
> blk_mq_complete_send_ipi():
> 
> 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.
> 
> This is expected after commit 4ccafe032005 ("block: unalign
> call_single_data in struct request"), which purposefully unaligned the
> structure to save space. Given that there is no real alignment
> requirement and there have been no reports of issues since that change,
> it should be safe to disable the warning for this one translation unit.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1328
> Link: https://lore.kernel.org/r/20210310182307.zzcbi5w5jrmveld4@archlinux-ax161/
> Link: https://lore.kernel.org/r/20210330230249.709221-1-jiancai@google.com/
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Reviewed-by: Guenter Roeck <linux@roeck-us.net>

> ---
>  block/Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/block/Makefile b/block/Makefile
> index 8d841f5f986f..d69ac0bd8e61 100644
> --- a/block/Makefile
> +++ b/block/Makefile
> @@ -10,6 +10,7 @@ obj-$(CONFIG_BLOCK) := bio.o elevator.o blk-core.o blk-sysfs.o \
>  			blk-mq-sysfs.o blk-mq-cpumap.o blk-mq-sched.o ioctl.o \
>  			genhd.o ioprio.o badblocks.o partitions/ blk-rq-qos.o
>  
> +CFLAGS_blk-mq.o := $(call cc-disable-warning, align-mismatch)
>  obj-$(CONFIG_BOUNCE)		+= bounce.o
>  obj-$(CONFIG_BLK_SCSI_REQUEST)	+= scsi_ioctl.o
>  obj-$(CONFIG_BLK_DEV_BSG)	+= bsg.o
> 
> base-commit: e49d033bddf5b565044e2abe4241353959bc9120
>