linux-kernel - Re: Re: [PATCH 1/5] ftrace: Synchronize variable setting with breakpoints

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Date:	Thu, 31 May 2012 11:29:20 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, Dave Jones <davej@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: Re: [PATCH 1/5] ftrace: Synchronize variable setting with
 breakpoints

On Fri, 2012-06-01 at 00:16 +0900, Masami Hiramatsu wrote:

> Perhaps, kprobes-*jump*-optimization may be better to handle it
> because the target probe is simultaneously working while
> optimizing (modifying code). This means that if someone hits
> breakpoint of such kprobe, it must be handled by kprobes, not
> only just tweaking IP address.

Yeah, ftrace is easier on what the breakpoint does. For kprobes, it's a
bit more invasive. Ftrace only deals with a nop, or a "trace-me" call.
As the modifications are being done, we always treat it as a nop until
the modification is complete.

With kprobes, the text_poke() is a bit more difficult, because it can't
treat the location as a nop, as what is changing actually performs some
task for the kernel. It requires that the breakpoint know what to do
with the text (out-of-line execution, and such). ftrace has the benefit
of just "skip this instruction" and return.

-- Steve

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