Messages in this thread Patch in this message | | | Date | Wed, 01 Aug 2012 16:51:39 +0200 | From | Sebastian Andrzej Siewior <> | Subject | Re: Q: user_enable_single_step() && update_debugctlmsr() |
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On 08/01/2012 04:31 PM, Oleg Nesterov wrote: > And I think you missed my point. I'll try again. Okay.
> 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. >
Sebastian
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