linux-kernel - Re: [PATCH] mm: thp: avoid uninitialized variable use

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Message-ID: <63b69d2b-1c1d-6f08-30a6-e996b6842000@alibaba-inc.com>
Date:   Sun, 17 Dec 2017 06:25:17 +0800
From:   "Yang Shi" <yang.s@...baba-inc.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.com>,
        Ingo Molnar <mingo@...nel.org>, Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: thp: avoid uninitialized variable use



On 12/16/17 4:24 AM, Arnd Bergmann wrote:
> On Fri, Dec 15, 2017 at 7:01 PM, Yang Shi <yang.s@...baba-inc.com> 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.

Thanks, I just upgraded gcc to 6.4 on my cetnos 7 machine. But, I ran 
into a build error with 4.15-rc3 kernel, but 4.14 is fine. I bisected to 
a commit in Makefile. I will email my bug report to the mailing list.

Regards,
Yang

> 
> See http://git.infradead.org/users/segher/buildall.git for a simple way
> to build toolchains suitable for building kernels in varying architectures
> and versions.
> 
>         Arnd
>