linux-kernel - Re: Q: user_enable_single_step() && update

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Message-ID: <50194750.2050601@linutronix.de>
Date:	Wed, 01 Aug 2012 17:12:16 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Roland McGrath <roland@...k.frob.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	ananth@...ibm.com, a.p.zijlstra@...llo.nl, mingo@...hat.com,
	srikar@...ux.vnet.ibm.com
Subject: Re: Q: user_enable_single_step() && update_debugctlmsr()

On 08/01/2012 05:01 PM, Oleg Nesterov wrote:
> On 08/01, Sebastian Andrzej Siewior wrote:
>> So a patch like
>> --- a/arch/x86/kernel/step.c
>> +++ b/arch/x86/kernel/step.c
>> @@ -173,8 +173,8 @@ static void enable_step(struct task_struct *child,
>> bool block)
>>                  unsigned long debugctl = get_debugctlmsr();
>>
>>                  debugctl |= DEBUGCTLMSR_BTF;
>> -               update_debugctlmsr(debugctl);
>>                  set_tsk_thread_flag(child, TIF_BLOCKSTEP);
>> +               update_debugctlmsr(debugctl);
>>          } else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP)) {
>>                  unsigned long debugctl = get_debugctlmsr();
>>
>> should fix the race
>
> No, I don't think it can fix something ;) or make any difference.

Why? You _first_ set the task flag followed by the CPU register. Now 
switch_to() would see the bit set and act.

>> and _yes_ I also would follow your suggestion to
>> remove this update_debugctlmsr() here since switch_to() should do this.
>
> Agreed, but once again, uprobes needs it if child == current (but we should
> move this code into the trivial helper). If we change (I hope) uprobes to
> avoid user_enable_single_step() we will export the helper.

Okay. Looking at TIF_NOTSC I see that it does a preempt_disable() while
playing with the bit. So this would be probably more obvious than
switching the order :)

>
> Oleg.

Sebastian
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