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

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Date:	Thu, 3 Dec 2015 09:28:00 +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>
Subject: Re: [PATCH] sched/core: Clear the root_domain cpumasks in
 init_rootdomain()


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

> Hi Peter,
> 
> On 12/03/2015 at 12:25 AM, Peter Zijlstra wrote:
> > On Wed, Dec 02, 2015 at 09:12:30PM +0800, Xunlei Pang wrote:
> >> Hi Peter,
> >>
> >> On 12/02/2015 at 08:34 PM, Peter Zijlstra wrote:
> >>> On Wed, Dec 02, 2015 at 07:52:59PM +0800, Xunlei Pang wrote:
> >>>> The patch cleans the garbage by using zalloc_cpumask_var()
> >>>> instead of alloc_cpumask_var() for root_domain::rto_mask
> >>>> allocation, thereby addressing the issues.
> >>> How did you notice this? Also do we want to do the same for the kmalloc
> >> When doing review.
> > Nice, will you be looking for similar issues elsewhere in the scheduler
> > too?
> 
> Sure :-)

Hm, is the alloc_cpumask_var() done in alloc_sched_domains() safe?

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);

I think alloc_cpumask_var() is a fundamentally unsafe or at least fragile 
operation, because the uninitialized variable bug will only happen on large CPU 
count kernels AFAICS - so it's inviting such bugs.

How about we rename alloc_cpumask_var() to alloc_cpumask_var_noinit() or at least 
__alloc_cpumask_var(), to make this property easier to see?

Thanks,

	Ingo
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