linux-kernel - Re: [PATCH] mm: initialize variable for mem_cgroup_end_page

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Date:	Thu, 30 Oct 2014 09:32:14 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	riel@...hat.com, hannes@...xchg.org, peterz@...radead.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] mm: initialize variable for mem_cgroup_end_page_stat

On 10/30/2014 04:27 AM, Michal Hocko wrote:
> On Wed 29-10-14 21:44:24, Sasha Levin wrote:
>> > Commit "mm: memcontrol: fix missed end-writeback page accounting" has changed
>> > the behaviour of mem_cgroup_begin_page_stat() to not always set the "locked"
>> > parameter.
>> > 
>> > We should initialize it at the callers to prevent garbage being used in a
>> > later call to mem_cgroup_end_page_stat().
> The contract is that if the returned memcg is non-NULL then the locked
> is always initialized. Nobody but mem_cgroup_end_page_stat should touch
> this variable and this function makes sure it uses it properly. Similar
> applies to flags which is initialized only if we really take the slow
> path (has a meaning only if locked == true).
> 
> So this is not really needed. Was this triggered by a compiler warning?

The problem is that you are attempting to read 'locked' when you call
mem_cgroup_end_page_stat(), so it gets used even before you enter the
function - and using uninitialized variables is undefined.

Yes, it's a compiler warning.


Thanks,
Sasha

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