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Date: Mon, 06 Aug 2012 18:14:45 +0200
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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To: Oleg Nesterov <oleg@redhat.com>
CC: Ingo Molnar <mingo@elte.hu>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Anton Arapov <anton@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
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        Roland McGrath <roland@hack.frob.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/2] ptrace: DEBUGCTLMSR_BTF fixes
References: <20120803162912.GA19767@redhat.com>
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On 08/03/2012 06:29 PM, Oleg Nesterov wrote:

> Assuming this series passes the review of course. And it would
> be really nice if someone reviews 2/2, I am not confident that
> I fully understand this DEBUGCTLMSR_BTF magic.

Here is how the processor disables the BTF:

| The processor clears the BTF flag when it generates a debug
| exception. The debugger must set the BTF flag before resuming program
| execution to continue single-stepping on branches.

This was a quote from "253668-039US, May 2011" page 16-17.

So I think __switch_to_extra() should set the bit before putting the
task on the CPU. If this bit is enabled on the wrong CPU then in will
remain set forever if single steeping has not been / will not be
enabled.

> Oleg.
>

Sebastian