linux-kernel - Re: [PATCH] sched/core: Clear the root_domain cpumasks in init

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Date:	Fri, 4 Dec 2015 09:09:01 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Xunlei Pang <xlpang@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] sched/core: Clear the root_domain cpumasks in
 init_rootdomain()


* Xunlei Pang <xlpang@...hat.com> wrote:

> > Hm, is the alloc_cpumask_var() done in alloc_sched_domains() safe?
> 
> Until now, I haven't found any other similar issues, but I will check further.
> 
> >
> > At least the usage pattern in init_sched_domains() looks unsafe:
> >
> >         doms_cur = alloc_sched_domains(ndoms_cur);
> >         if (!doms_cur)
> >                 doms_cur = &fallback_doms;
> >         cpumask_andnot(doms_cur[0], cpu_map, cpu_isolated_map);

So is this pattern in init_sched_domains() correct, for OFFSTACK=y?

It looks wrong to me, as alloc_sched_domains() allocates an uninitialized cpumask 
via alloc_cpumask_var() and returns it:

cpumask_var_t *alloc_sched_domains(unsigned int ndoms)
{
        int i;
        cpumask_var_t *doms;

        doms = kmalloc(sizeof(*doms) * ndoms, GFP_KERNEL);
        if (!doms)
                return NULL;
        for (i = 0; i < ndoms; i++) {
                if (!alloc_cpumask_var(&doms[i], GFP_KERNEL)) {
                        free_sched_domains(doms, i);
                        return NULL;
                }
        }
        return doms;
}

and then this code:

> >         cpumask_andnot(doms_cur[0], cpu_map, cpu_isolated_map);

uses it without first clearing it.

So is this another such bug, or am I missing something?

Thanks,

	Ingo
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