Messages in this thread | | | Date | Wed, 01 Aug 2012 20:46:44 +0200 | From | Sebastian Andrzej Siewior <> | Subject | Re: Q: user_enable_single_step() && update_debugctlmsr() |
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On 08/01/2012 05:14 PM, Oleg Nesterov wrote: > On 08/01, Sebastian Andrzej Siewior wrote: >> >> 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 > > Yes, and this task is "child". > >> followed by the CPU register. Now >> switch_to() would see the bit set and act. > > child sleeps and doesn't participate in switch_to(). Debugger and another > (unrelated) task do.
This is confusing.
In order to allow the debugger to ptrace()->enable_blockstep() the child has to be stopped/traced. We switch X86_EFLAGS_TF in child's regs and enable DEBUGCTLMSR_BTF for the debugger which is wrong. If we quit to userspace then the CPU on which the debugger runs has DEBUGCTLMSR_BTF. If the tracee task runs on the same then nothing happens, the bit remains set. If the tracee happens to run on a different CPU then switch_to() will enable the DEBUGCTLMSR_BTF bit for the debugger's CPU and switch_to() will enable it also on the other CPU. I added a few printks in the source and I see output that __switch_to_xtra() enables the bit as well as in enable_single() for debugger's CPU. I didn't find where the single step is disabled for the tracee. I haven't notice this in __switch_to_xtra() nor or in disable_single_step().
> Oleg. >
Sebastian
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