powerpc/32: Avoid miscompilation w/GCC 4.6.3 - don't inline copy_to/from_user()

[Michael Ellerman]

On Mon, 2017-06-26 at 13:34:15 UTC, Michael Ellerman wrote:
> Larry Finger reported that his Powerbook G4 was no longer booting with v4.12-rc,
> userspace was up but giving weird errors such as:
> 
>   udevd[64]: starting version 175
>   udevd[64]: Unable to receive ctrl message: Bad address.
>   modprobe: chdir(4.12-rc1): No such file or directory
> 
> He bisected the problem to commit 3448890c32c3 ("powerpc: get rid of zeroing,
> switch to RAW_COPY_USER").
> 
> Al identified that the problem is actually a miscompilation by GCC 4.6.3, which
> is exposed by the above commit.
> 
> Al also pointed out that inlining copy_to/from_user() is probably of little or
> no benefit, which is correct. Using Anton's copy_to_user benchmark, with a
> pathological single byte copy, we see a small increase in performance
> by *removing* inlining:
> 
>   Before (inlined):
>   # time ./copy_to_user -w -l 1 -i 10000000	( x 3 )
>   real	0m22.063s
>   real	0m22.059s
>   real	0m22.076s
> 
>   After:
>   # time ./copy_to_user -w -l 1 -i 10000000	( x 3 )
>   real	0m21.325s
>   real	0m21.299s
>   real	0m21.364s
> 
> So as a small performance improvement and to avoid the miscompilation, drop
> inlining copy_to/from_user() on 32-bit.
> 
> Fixes: 3448890c32c3 ("powerpc: get rid of zeroing, switch to RAW_COPY_USER")
> Reported-by: Larry Finger <Larry.Finger at lwfinger.net>
> Suggested-by: Al Viro <viro at zeniv.linux.org.uk>
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>

Applied to powerpc fixes.

https://git.kernel.org/powerpc/c/d6bd8194e2867e85ac2de63486d3b8

cheers