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SubjectRe: [PATCH 4/7] ptrace: Partly fix set_task_blockstep()->update_debugctlmsr() logic
* Oleg Nesterov | 2012-09-03 17:26:09 [+0200]:

>Afaics the usage of update_debugctlmsr() and TIF_BLOCKSTEP in
>step.c was always very wrong.
>1. update_debugctlmsr() was simply unneeded. The child sleeps
> TASK_TRACED, __switch_to_xtra(next_p => child) should notice
> TIF_BLOCKSTEP and set/clear DEBUGCTLMSR_BTF after resume if
> needed.
>2. It is wrong. The state of DEBUGCTLMSR_BTF bit in CPU register
> should always match the state of current's TIF_BLOCKSTEP bit.
>3. Even get_debugctlmsr() + update_debugctlmsr() itself does not
> look right. Irq can change other bits in MSR_IA32_DEBUGCTLMSR
> register or the caller can be preempted in between.

ptrace and uprobe are calling this function from process context. As
long as you have here get_cpu() instead of local_irq_disable() you should
be safe here.
The only user that is touching this bits in irq context is perf. perf
uses raw_local_irqsave() (raw_* most likely due to -RT). I have no idea
what you can against NMI unless not touching the register in NMI


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