linux-kernel - Re: [tip:sched/urgent] sched/fair: Avoid using uninitialized variable in preferred_group

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Date:	Wed, 28 Jan 2015 14:46:19 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	<mingo@...nel.org>
Cc:	<peterz@...radead.org>, <tglx@...utronix.de>,
	<torvalds@...ux-foundation.org>, <riel@...hat.com>,
	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [tip:sched/urgent] sched/fair:  Avoid using uninitialized
 variable in preferred_group_nid()

>>> On 28.01.15 at 15:29, <tipbot@...or.com> wrote:
> Commit-ID:  81907478c4311a679849216abf723999184ab984
> Gitweb:     
> http://git.kernel.org/tip/81907478c4311a679849216abf723999184ab984 
> Author:     Jan Beulich <JBeulich@...e.com>
> AuthorDate: Fri, 23 Jan 2015 08:25:38 +0000
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Wed, 28 Jan 2015 13:14:12 +0100
> 
> sched/fair: Avoid using uninitialized variable in preferred_group_nid()
> 
> 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.
> 
> [ mingo: changed the solution to zero initialization: uninitialized_var()
>   needs to die, as it's an actively dangerous construct: if in the future
>   a known-proven-good piece of code is changed to have a true, buggy
>   uninitialized variable, the compiler warning is then supressed...

But you went farther than that: You also dropped the breaking
out of the outer loop. Yet that has - beyond the fixing of the bug
here - the desirable effect of not continuing for perhaps many
iterations when nothing new can ever be found anymore.

Jan

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