linux-kernel - Re: [PATCH] sched_groups are expected to be circular linked list, make it so right after allocation

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Message-ID: <4FAA5BFB.40309@redhat.com>
Date:	Wed, 09 May 2012 13:58:51 +0200
From:	Igor Mammedov <imammedo@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Jiang Liu <liuj97@...il.com>, linux-kernel@...r.kernel.org,
	mingo@...nel.org, pjt@...gle.com, tglx@...utronix.de,
	seto.hidetoshi@...fujitsu.com
Subject: Re: [PATCH] sched_groups are expected to be circular linked list,
 make it so right after allocation

On 05/09/2012 01:52 PM, Peter Zijlstra wrote:
> On Wed, 2012-05-09 at 13:44 +0200, Igor Mammedov wrote:
>> This patch fixes only build_sched_groups path, but there is another fail path
>> that results in below OOPS.
>> build_overlap_sched_groups() may exit without setting groups and later it will crash
>> init_sched_groups_power as well.
>
> if that allocation fails? Or is there another fail path?

build_overlap_sched_groups(struct sched_domain *sd, int cpu)
...
                 if (cpumask_test_cpu(cpu, sg_span))
                         groups = sg;
...

above test fails and leaves local var groups set to NULL
and before exit there is:

   sd->groups = groups;

which resets sd->groups to NULL and I'm not sure if it is correct at all to skip this
assignment if groups == NULL.

>
>>   But I just don't know how to fix it, so I've just
>> posted partial fix that reduces crash frequency.
>
>
>> And I have to admit that
>> cpu_active_mask and siblings map are busted but we either should not exit from builder
>> funcs with NULL group or BUG there if it is impossible to come-up with sane group
>> for insane domain span.
>
> I'm perfectly OK with taking the machine down, provided we can output
> useful messages as to what is broken first..

-- 
-----
  Igor
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