linux-kernel - Re: [PATCH 4/7] ptrace: Partly fix set_task_blockstep()->update

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Message-ID: <20120910172736.GA14168@redhat.com>
Date:	Mon, 10 Sep 2012 19:27:36 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Roland McGrath <roland@...k.frob.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] ptrace: Partly fix
	set_task_blockstep()->update_debugctlmsr() logic

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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