From: Peter Zijlstra <peterz@infradead.org>
To: Jan Beulich <JBeulich@suse.com>
Cc: mingo@redhat.com, linux-kernel@vger.kernel.org,
Rik van Riel <riel@redhat.com>
Subject: Re: sched/fair: avoid using uninitialized variable in preferred_group_nid()
Date: Fri, 23 Jan 2015 10:54:49 +0100 [thread overview]
Message-ID: <20150123095449.GL2896@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <54C2139202000078000588F7@mail.emea.novell.com>
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).
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> Cc: Rik van Riel <riel@redhat.com>
> ---
> kernel/sched/fair.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> --- 3.19-rc5/kernel/sched/fair.c
> +++ 3.19-rc5-sched-fair-preferred-group-nid/kernel/sched/fair.c
> @@ -1730,7 +1730,7 @@ static int preferred_group_nid(struct ta
> nodes = node_online_map;
> for (dist = sched_max_numa_distance; dist > LOCAL_DISTANCE; dist--) {
> unsigned long max_faults = 0;
> - nodemask_t max_group;
> + nodemask_t uninitialized_var(max_group);
> int a, b;
>
> /* Are there nodes at this distance from each other? */
> @@ -1764,6 +1764,8 @@ static int preferred_group_nid(struct ta
> }
> }
> /* Next round, evaluate the nodes within max_group. */
> + if (!max_faults)
> + break;
> nodes = max_group;
> }
> return nid;
Yes I think you're right; there is no guarantee the faults sum will be
greater than 0, and therefore we might not trigger the assignment and
end up with uninitialized use.
next prev parent reply other threads:[~2015-01-23 9:54 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 [this message]
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
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20150123095449.GL2896@worktop.programming.kicks-ass.net \
--to=peterz@infradead.org \
--cc=JBeulich@suse.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=riel@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).