Re: [RFC PATCH 02/10] ftrace: Ensure tracing has really stoppedbefore leaving unregister_ftrace_graph

From: Steven Rostedt
Date: Thu Jan 21 2010 - 20:51:59 EST

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).

> 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.

-- Steve

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