linux-kernel - Re: sched/fair: avoid using uninitialized variable in preferred_group

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Message-Id: <54C229F902000078000589FD@mail.emea.novell.com>
Date:	Fri, 23 Jan 2015 10:01:13 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Peter Zijlstra" <peterz@...radead.org>
Cc:	<mingo@...hat.com>, "Rik van Riel" <riel@...hat.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: sched/fair: avoid using uninitialized variable in
 preferred_group_nid()

>>> On 23.01.15 at 10:54, <peterz@...radead.org> wrote:
> On Fri, Jan 23, 2015 at 08:25:38AM +0000, Jan Beulich wrote:
>> At least some gcc versions - validly afaict - warn about potentially
>> using max_group uninitialized: There's no way the compiler can prove
>> that the body of the conditional where it and max_faults get set/
>> updated gets executed; in fact, without knowing all the details of
>> other scheduler code, I can't prove this either.
>> 
>> Generally the necessary change would appear to be to clear max_group
>> prior to entering the inner loop, and break out of the outer loop when
>> it ends up being all clear after the inner one. This, however, seems
>> inefficient, and afaict the same effect can be achieved by exiting the
>> outer loop when max_faults is still zero after the inner loop. For the
>> compiler's sake, mark max_group uninitialized, as we're now able to
>> prove it's not actually being used uninitalized anymore.
> 
> 
> Yes this is somewhat challenging. What compiler version in specific did
> you get this warning wiht? I cannot remember seeing it with whatever it
> is I use (4.7-4.9 it seems).

SLE11 SP3's gcc (4.3.4 based).

Jan

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