linux-kernel - Re: [PATCH 0/3] ring-buffer: less locking and only disable preemption

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Date:	Sat, 4 Oct 2008 16:44:23 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH 0/3] ring-buffer: less locking and only disable
	preemption


* Steven Rostedt <rostedt@...dmis.org> wrote:

> The dynamic function tracer is another issue. The problem with NMIs 
> has nothing to do with locking, or corrupting the buffers. It has to 
> do with the dynamic code modification.  Whenever we modify code, we 
> must guarantee that it will not be executed on another CPU.
> 
> Kstop_machine serves this purpose rather well. We can modify code 
> without worrying it will be executed on another CPU, except for NMIs. 
> The problem now comes where an NMI can come in and execute the code 
> being modified. That's why I put in all the notrace, lines. But it 
> gets difficult because of nmi_notifier can call all over the kernel.  
> Perhaps, we can simply disable the nmi-notifier when we are doing the 
> kstop_machine call?

that would definitely be one way to reduce the cross section, but not 
enough i'm afraid. For example in the nmi_watchdog=2 case we call into 
various lapic functions and paravirt lapic handlers which makes it all 
spread to 3-4 paravirtualization flavors ...

sched_clock()'s notrace aspects were pretty manageable, but this in its 
current form is not.

	Ingo
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