RE: [PATCH] blk-mq: fix alignment mismatch.

[David Laight]

From: Nathan Chancellor
> Sent: 31 March 2021 00:30
> 
> Hi Jian,
> 
> On Tue, Mar 30, 2021 at 04:02:49PM -0700, Jian Cai wrote:
> > This fixes the mismatch of alignments between csd and its use as an
> > argument to smp_call_function_single_async, which causes build failure
> > when -Walign-mismatch in Clang is used.
> >
> > Link:
> > http://crrev.com/c/1193732
> >
> > Suggested-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> > Signed-off-by: Jian Cai <jiancai@xxxxxxxxxx>
> 
> Thanks for the patch. This is effectively a revert of commit
> 4ccafe032005 ("block: unalign call_single_data in struct request"),
> which I had brought up in this thread:
> 
> https://lore.kernel.org/r/20210310182307.zzcbi5w5jrmveld4@archlinux-ax161/
> 
> This is obviously a correct fix, I am not just sure what the impact to
> 'struct request' will be.

If the structure is allocated on-stack then aligning it
requires the compiler generate the rather horrid
'double stack frame' for the function.

Possibly the unaligned 'struct' should be used by all
the code except for a few places where it makes sense
to allocate an aligned item?

	David

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