linux-kernel - Re: [PATCH] arm64: ftrace: stop using kstop

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Date:	Thu, 3 Dec 2015 15:09:26 +0000
From:	Will Deacon <will.deacon@....com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	libin <huawei.libin@...wei.com>, mingo@...hat.com,
	catalin.marinas@....com, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, guohanjun@...wei.com,
	dingtianhong@...wei.com
Subject: Re: [PATCH] arm64: ftrace: stop using kstop_machine to
 enable/disable tracing

On Thu, Dec 03, 2015 at 10:05:25AM -0500, Steven Rostedt wrote:
> On Thu, 3 Dec 2015 09:38:21 +0000
> Will Deacon <will.deacon@....com> wrote:
> > I think you're missing the case where the instruction changes under our
> > feet after we've read it but before we've replaced it (e.g. due to module
> > unloading). I think that's why ftrace_modify_code has the comment about
> > lack of locking thanks to stop_machine.
> 
> Note, ftrace has a module notifier that is called when a module is
> being unloaded and before the text goes away. This code grabs the
> ftrace_lock mutex and removes the module functions from the ftrace
> list, such that it will no longer do any modifications to that module's
> text.
> 
> The update to make functions be traced is done under the ftrace_lock
> mutex as well.
> 
> You do not need to worry about module text disappearing from
> underneath you while you do your modifications.

Good.

> Now, if there's comments that suggest otherwise, they need to be
> updated.

Yeah, I think the comments on x86 and arm64 are out of date. They also
mention the freeing of __init sections -- is that still a concern?

Will
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