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    SubjectRe: 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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