Re: [PATCH 4/7] ptrace: Partly fixset_task_blockstep()->update_debugctlmsr() logic

[Oleg Nesterov]

Sebastian, I am replying to my message because I removed your email
by mistake. Fortunately I can see it on marc.info...

> 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.

local_irq_disable() looks more safe. We can have new users playing
with MSR_IA32_DEBUGCTLMSR from irq.

> perf
> uses raw_local_irqsave() (raw_* most likely due to -RT).

This is completely irrelevant, we alrady discussed this.

> I have no idea
> what you can against NMI unless not touching the register in NMI
> context.

Neither me, and this is documented in the changelog:

	And afaics there is yet another problem: perf can play with
	MSR_IA32_DEBUGCTLMSR from nmi, this obviously means that even
	__switch_to_xtra() has problems.

and please note __switch_to_xtra() above, it has the same problem
by the same reason.

Oleg.

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