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SubjectRe: [PATCH v6 4/7] arm64: Add ftrace support
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On Thu, 2014-03-13 at 18:37 +0000, Will Deacon wrote:
>
> > No, return_hooker is consistent with all the other archs. Hey, it's a
> > rugby position! Note, which I was when I played. ;-)
>
> Hehe, in which case your children will be able to execute that line of
> ftrace!
>

My kids already know I was :-)

> > >
> > > > + trace.func = self_addr;
> > >
> > > in kernel/ftrace/ftrace.c there's an smb_wmb() between setting up
> > > tracing_graph_pause and setting the ret_stack with a comment saying:
> > >
> > > /* Make sure the tasks see the -1 first: */
> > >
> > > Why don't we have a corresponding read-barrier here?
> >
> > The corresponding rmb is in kernel/trace/trace_function_graph
> > ftrace_push_return_trace().
>
> Okey doke then, I guess we don't really care about tracing_graph_pause
> getting out-of-sync with curr_ret_stack.

Yep, the tracing_graph_pause is more to prevent crazy stuff happening
when an interrupt comes in. So it is just local cpu context, and doesn't
have issues with other CPUS. If it takes a bit for the task to see it go
to zero, the worse thing is that you might miss a few tracing functions
until that gets synced. Which will probably happen by the time tracing
is fully activated anyway.

-- Steve




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