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Date: Fri, 3 Aug 2012 19:38:30 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Anton Arapov <anton@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Roland McGrath <roland@hack.frob.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] ptrace: fix
	set_task_blockstep()->update_debugctlmsr() logic
Message-ID: <20120803173830.GA22687@redhat.com>
References: <20120803162954.GA19806@redhat.com> <501BFF95.8040202@linutronix.de>
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On 08/03, Sebastian Andrzej Siewior wrote:
>
> On 08/03/2012 06:29 PM, Oleg Nesterov wrote:
>> diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
>> index afa60db..636402e 100644
>> --- a/arch/x86/kernel/step.c
>> +++ b/arch/x86/kernel/step.c
>> @@ -166,12 +166,18 @@ static void set_task_blockstep(struct task_struct *task, bool on)
>>   	else
>>   		clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
>>
>> +	if (task != current)
>> +		return;
>> +
>> +	/* ensure irq/preemption can't change debugctl in between */
>> +	local_irq_disable();
>>   	debugctl = get_debugctlmsr();
>>   	if (on)
>>   		debugctl |= DEBUGCTLMSR_BTF;
>>   	else
>>   		debugctl&= ~DEBUGCTLMSR_BTF;
>>   	update_debugctlmsr(debugctl);
>> +	local_irq_enable();
>
> wouldn't preempt_disable() be enough?

preempt_disable() can't help if interrupt handler changes
other bits in between?

Oleg.