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

[Oleg Nesterov <oleg@redhat.com>]

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.

> 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.

Oleg.