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

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Date:	Mon, 10 Sep 2012 18:57:17 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	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

* 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
context.

Sebastian
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