linux-kernel - Re: [RFC][PATCH 0/4] ftrace: Use -mfentry when supported (this is for x86

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Message-ID: <1298070174.23343.987.camel@gandalf.stny.rr.com>
Date:	Fri, 18 Feb 2011 18:02:54 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Dominique Toupin <dominique.toupin@...csson.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"2nddept-manager@....hitachi.co.jp" 
	<2nddept-manager@....hitachi.co.jp>
Subject: Re: [RFC][PATCH 0/4] ftrace: Use -mfentry when supported (this is
 for x86_64 right now)

On Fri, 2011-02-18 at 14:45 -0800, H. Peter Anvin wrote:

> We should also be able to use the breakpoint hack to avoid holding all
> the CPUs.  They still need to be interrupted, but that skips the
> rendezvous operation.

As this is about the ftrace code, I'm in the process of analyzing and
updating how the function tracer works. I can look to see if I can
design it so we don't have to always use stop_machine() if a breakpoint
method is in place.

Basically what is needed is to convert a "nop" into a "call" or maybe
the other way around, safely.

Now is it safe to insert a breakpoint (usually a byte I believe), modify
the rest of the instruction and then replace the breakpoint to the new
code? Since the instruction that is being replaced or the instruction
being added is always a nop, the breakpoint handler needs to do nothing
but return to the location after the nop/call.

Is there any synchronization that needs to be done when doing this? Or
can it just be:

	insert_breakpoint();
	update_instruction();
	remove_breakpoint();

Because we need to do this for 22,000 calls in a row.

-- Steve

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