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    SubjectRe: Q: user_enable_single_step() && update_debugctlmsr()
    On 08/01/2012 04:31 PM, Oleg Nesterov wrote:
    > And I think you missed my point. I'll try again.

    > We have the GDB process and the (stopped) tracee T. And we have
    > another task X which have TIF_SINGLESTEP but not TIF_BLOCKSTEP.
    > To simplify, suppose that X is already TASK_RUNNING but not on rq.
    > GDB does ptrace(PTRACE_SINGLEBLOCK, T). This sets X->TIF_BLOCKSTEP.
    > Now suppose that GDB is preempted right after it does
    > update_debugctlmsr(), and the scheduler choses X as the next task.
    > Both GDB and X do not have TIF_BLOCKSTEP, so __switch_to_extra()
    > does not update DEBUGCTLMSR_BTF.
    > X returns to the user-mode with TIF_SINGLESTEP and TIF_BLOCKSTEP,
    > the latter is wrong.
    > No?

    Yup, correct, you are right sir. Thank you for trying so hard to make
    me see this :)

    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 and _yes_ I also would follow your suggestion to
    remove this update_debugctlmsr() here since switch_to() should do this.
    > Oleg.


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