From: Peter Zijlstra <peterz@infradead.org>
To: Jan Beulich <JBeulich@suse.com>
Cc: mingo@kernel.org, tglx@linutronix.de,
torvalds@linux-foundation.org, riel@redhat.com,
linux-kernel@vger.kernel.org, hpa@zytor.com
Subject: Re: [tip:sched/urgent] sched/fair: Avoid using uninitialized variable in preferred_group_nid()
Date: Wed, 28 Jan 2015 16:37:54 +0100 [thread overview]
Message-ID: <20150128153754.GG23038@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <54C9044B020000780005A5E6@mail.emea.novell.com>
On Wed, Jan 28, 2015 at 02:46:19PM +0000, Jan Beulich wrote:
> >>> On 28.01.15 at 15:29, <tipbot@zytor.com> wrote:
> > Commit-ID: 81907478c4311a679849216abf723999184ab984
> > Gitweb:
> > http://git.kernel.org/tip/81907478c4311a679849216abf723999184ab984
> > Author: Jan Beulich <JBeulich@suse.com>
> > AuthorDate: Fri, 23 Jan 2015 08:25:38 +0000
> > Committer: Ingo Molnar <mingo@kernel.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.
next prev parent reply other threads:[~2015-01-29 3:38 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-01-23 8:25 sched/fair: avoid using uninitialized variable in preferred_group_nid() Jan Beulich
2015-01-23 9:54 ` Peter Zijlstra
2015-01-23 10:01 ` Jan Beulich
2015-01-28 14:29 ` [tip:sched/urgent] sched/fair: Avoid " tip-bot for Jan Beulich
2015-01-28 14:46 ` Jan Beulich
2015-01-28 15:37 ` Peter Zijlstra [this message]
2015-02-09 8:21 ` Ingo Molnar
2015-02-09 11:37 ` Peter Zijlstra
2015-02-09 11:41 ` Jan Beulich
2015-02-18 17:10 ` [tip:sched/core] sched/numa: Avoid some pointless iterations tip-bot for Jan Beulich
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