Re: [PATCH] mm: thp: avoid uninitialized variable use

From: Arnd Bergmann
Date: Sat Dec 16 2017 - 07:24:12 EST

On Fri, Dec 15, 2017 at 7:01 PM, Yang Shi <yang.s@xxxxxxxxxxxxxxx> wrote:
> On 12/15/17 4:51 AM, Arnd Bergmann wrote:
>> When the down_read_trylock() fails, 'vma' has not been initialized
>> yet, which gcc now warns about:
>> mm/khugepaged.c: In function 'khugepaged':
>> mm/khugepaged.c:1659:25: error: 'vma' may be used uninitialized in this
>> function [-Werror=maybe-uninitialized]
> Arnd,
> Thanks for catching this. I'm wondering why my test didn't catch it. It
> might be because my gcc is old. I'm using gcc 4.8.5 on centos 7.

Correct, gcc-4.8 and earlier have too many false-positive warnings with
-Wmaybe-uninitialized, so we turn it off on those versions. 4.9 is much
better here, but I'd recommend using gcc-6 or gcc-7 when you upgrade,
they have a much better set of default warnings besides producing better
binary code.

See for a simple way
to build toolchains suitable for building kernels in varying architectures
and versions.