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

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Message-ID: <20150128153754.GG23038@twins.programming.kicks-ass.net>
Date:	Wed, 28 Jan 2015 16:37:54 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	mingo@...nel.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 Wed, Jan 28, 2015 at 02:46:19PM +0000, Jan Beulich wrote:
> >>> 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.

That break is indeed desired. The 'problem' it fixes is that when
group_faults() returns 0, faults will be 0, which will not > max_faults,
and therefore we will not set max_group.

Without that break, we'll now set nodes to NODE_MASK_NONE, which will
mean the for_each_node(a, nodes) loop will NOP and our dist loop will
iterate pointlessly.
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