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Date: Mon, 18 Feb 2013 21:19:26 -0500
From: Sasha Levin <sasha.levin@oracle.com>
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To: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@elte.hu>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Dave Jones <davej@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: sched: BUG in load_balance
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On 02/18/2013 08:26 PM, Steven Rostedt wrote:
> On Mon, 2013-02-18 at 19:55 -0500, Sasha Levin wrote:
>> Hi all,
>>
>> While fuzzing with trinity inside a KVM tools guest, running today's -next,
>> I've stumbled on the following spew.
>>
>> I've cc'ed Steven Rostedt since the culprit looks like "sched: Enable
>> interrupts in idle_balance()".
> 
> You're correct. Interrupts are ok but softirqs must still be disabled.
> 
> The following patch should work.
> 
> -- Steve
> 
> Only compiled tested:
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0fcdbff..a31174c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5222,9 +5222,9 @@ void idle_balance(int this_cpu, struct rq *this_rq)
>  	update_rq_runnable_avg(this_rq, 1);
>  
>  	/*
> -	 * Drop the rq->lock, but keep preempt disabled.
> +	 * Drop the rq->lock, but keep softirqs disabled.
>  	 */
> -	preempt_disable();
> +	local_bh_disable();
>  	raw_spin_unlock_irq(&this_rq->lock);
>  
>  	update_blocked_averages(this_cpu);
> @@ -5253,7 +5253,7 @@ void idle_balance(int this_cpu, struct rq *this_rq)
>  	rcu_read_unlock();
>  
>  	raw_spin_lock_irq(&this_rq->lock);
> -	preempt_enable();
> +	local_bh_enable();

I have to admit, I'm slightly confused with the patch: there's a raw_spin_lock_irq()
followed by local_bh_enable(). afaik it's illegal to call local_bh_enable() with
interrupts disabled.


Thanks,
Sasha