linux-kernel - Re: [PATCH] mm: fix uninitialized variable warnings

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Date:   Mon, 5 Nov 2018 16:35:08 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Jan Kara <jack@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Wang Long <wanglong19@...tuan.com>,
        Matthew Wilcox <willy@...radead.org>,
        Dave Chinner <dchinner@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] mm: fix uninitialized variable warnings

On 11/5/18, Jan Kara <jack@...e.cz> wrote:
> On Fri 02-11-18 16:31:06, Arnd Bergmann wrote:
>> In a rare randconfig build, I got a warning about possibly uninitialized
>> variables:
>>
>> mm/page-writeback.c: In function 'balance_dirty_pages':
>> mm/page-writeback.c:1623:16: error: 'writeback' may be used uninitialized
>> in this function [-Werror=maybe-uninitialized]
>>     mdtc->dirty += writeback;
>>                 ^~
>> mm/page-writeback.c:1624:4: error: 'filepages' may be used uninitialized
>> in this function [-Werror=maybe-uninitialized]
>>     mdtc_calc_avail(mdtc, filepages, headroom);
>>     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> mm/page-writeback.c:1624:4: error: 'headroom' may be used uninitialized in
>> this function [-Werror=maybe-uninitialized]
>>
>> The compiler evidently fails to notice that the usage is in dead code
>> after 'mdtc' is set to NULL when CONFIG_CGROUP_WRITEBACK is disabled.
>> Adding an IS_ENABLED() check makes this clear to the compiler.
>>
>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>
> I'm surprised the compiler was not able to infer this since:
>
> struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
>                                                      &mdtc_stor : NULL;
>
> and if CONFIG_CGROUP_WRITEBACK is disabled, mdtc_valid() is defined to
> 'false'.  But possibly the function is just too big and the problematic
> condition is in the loop so maybe it all confuses the compiler too much.

On second thought, I suspect this started with the introduction of
CONFIG_NO_AUTO_INLINE in linux-next. That also caused a similar
issue in 28 other files that I patched later. I wrote this patch before I
saw the others, and then didn't make the connection.

Let's drop the patch for now, and decide what we want to do for the
others. I fixed those by adding 'inline' markers for whatever
function needed it.

       Arnd