linux-kernel - Re: rcu_dereference() without protection in select_task_rq

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Date:	Mon, 15 Feb 2010 09:36:45 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: rcu_dereference() without protection in select_task_rq_fair()

On Mon, Feb 15, 2010 at 10:12:06AM +0100, Peter Zijlstra wrote:
> On Sun, 2010-02-14 at 09:04 -0800, Paul E. McKenney wrote:
> 
> > OK, but doesn't the "preempt_count() != 0" that is in the current version
> > of rcu_read_lock_sched_held() already cover this check?
> 
> Hmm, yes it should.
> 
> > In other words, I believe that I have located a usage of for_each_domain()
> > that violates the rule that it may only be called within preempt-disabled
> > sections.
> 
> >From the trace:
> 
> >  [<ffffffff81033ec4>] select_task_rq_fair+0xc1/0x686
> >  [<ffffffff810353f4>] wake_up_new_task+0x1e/0x13e
> 
> Which reads like:
> 
> void wake_up_new_task(...)
> {
> 	...
> 
>         int cpu __maybe_unused = get_cpu();
> 
> #ifdef CONFIG_SMP
>         /*
>          * Fork balancing, do it here and not earlier because:
>          *  - cpus_allowed can change in the fork path
>          *  - any previously selected cpu might disappear through hotplug
>          *
>          * We still have TASK_WAKING but PF_STARTING is gone now, meaning
>          * ->cpus_allowed is stable, we have preemption disabled, meaning
>          * cpu_online_mask is stable.
>          */
>         cpu = select_task_rq(p, SD_BALANCE_FORK, 0);
>         set_task_cpu(p, cpu);
> #endif
> 
> 	...
> 
> 	put_cpu()
> }
> 
> I cannot see how we can get there without preemption disabled.

Interesting point.  I have seen this but once.  If it reproduces, I will
instrument the code path and see if I can track it down.

							Thanx, Paul
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