linux-kernel - [PATCH] block: Disable -Walign-mismatch for blk-mq.c

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Message-Id: <20210408194458.501617-1-nathan@kernel.org>
Date:   Thu,  8 Apr 2021 12:44:58 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Jian Cai <jiancai@...gle.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Christopher Di Bella <cjdb@...gle.com>,
        Manoj Gupta <manojgupta@...gle.com>,
        Luis Lozano <llozano@...gle.com>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH] block: Disable -Walign-mismatch for blk-mq.c

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@...nel.org>
---
 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
-- 
2.31.1.189.g2e36527f23