linux-kernel - Re: [RFC PATCH 02/10] ftrace: Ensure tracing has really stopped before leaving unregister_ftrace

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Date:	Fri, 22 Jan 2010 03:04:05 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>
Subject: Re: [RFC PATCH 02/10] ftrace: Ensure tracing has really stopped
	before leaving unregister_ftrace_graph

On Thu, Jan 21, 2010 at 08:51:48PM -0500, Steven Rostedt wrote:
> On Fri, 2010-01-22 at 02:16 +0100, Frederic Weisbecker wrote:
> > When we run under dynamic tracing, we know that after calling
> > unregister_ftrace_graph(), tracing has really stopped because of
> > the hot patching and use of stop_machine().
> 
> This is incorrect. Even after unregister_ftrace_graph() with
> stop_machine(), we still have no guarantee that a call back is not being
> called. This is the reason I use sub tracing instead of NULLs. The call
> to the trace function could have been loaded in a register and then
> preempted. Even after stop_machine() that trace function can be called.
> This is also the reason that I never let modules add hooks to the
> function tracer (although I can easily make a wrapper to do so).



Ah, you are utterly right! I forgot about all that. And looks like
nothing can easily help this.

I just dream about a magic synchronize_trace().

 
> > 
> > But in static tracing case, we only set stub callbacks. This is
> > not sufficient on archs that have weak memory ordering to assume
> > the older callbacks won't be called right after we leave
> > unregister_ftrace_graph().
> > 
> > Insert a read/write memory barrier in the end of
> > unregister_ftrace_graph() so that the code that follow can safely
> > assume tracing has really stopped. This can avoid its older tracing
> > callbacks to perform checks about various states like ensuring
> > needed buffers have been allocated, etc...
> 
> There's no guarantee, even with a smp_mb() that a trace function will
> not be called after being unregistered.



Yeah, indeed...

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