linux-kernel - Re: [PATCH 2/5] ftrace: Use breakpoint method to update ftrace caller

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Date:	Thu, 31 May 2012 13:18:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"H. Peter Anvin" <hpa@...or.com>, Dave Jones <davej@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 2/5] ftrace: Use breakpoint method to update ftrace
 caller

On Wed, 2012-05-30 at 21:28 -0400, Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@...hat.com>
> 
> On boot up and module load, it is fine to modify the code directly,
> without the use of breakpoints. This is because boot up modification
> is done before SMP is initialized, thus the modification is serial,
> and module load is done before the module executes.
> 
> But after that we must use a SMP safe method to modify running code.
> 
> This has been done to change the nops at all the functions, but
> the change of the ftrace callback handler itself was still using a
> direct modification. If tracing was enabled and the function callback
> was changed then another CPU could fault if it was currently calling
> the original callback. This modification must use the breakpoint method
> too.
> 
> Note, the direct method is still used for boot up and module load.

The changelog isn't clear if this is a fix or optimization. I suspect
the latter.

Still, you're now re-inventing text_poke() and text_poke_early().

Why are you keeping all this inside of ftrace?


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