Re: Q: user_enable_single_step() && update_debugctlmsr()

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

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