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SubjectRe: [PATCH 05/19] x86/dumpstack: fix function graph tracing stack dump reliability issues
On Fri, Jul 29, 2016 at 10:20:36PM -0400, Steven Rostedt wrote:
> On Fri, 29 Jul 2016 19:50:59 -0500
> Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> > BTW, it would be really nice if ftrace_graph_ret_addr() were idempotent
> > so we could get the "real" return address without having to pass in a
> > state variable.
> >
> > For example we could add an "unsigned long *retp" pointer to
> > ftrace_ret_stack, which points to the return address on the stack. Then
> > we could get rid of the index state variable in ftrace_graph_ret_addr,
> > and also then there would never be a chance of the stack dump getting
> > out of sync with the ret_stack.
> >
> > What do you think?
> >
>
> I don't want to extend ret_stack as that is allocated 50 of these
> structures for every task. That said, we have the "fp" field that's
> used to check for frame pointer corruption when mcount is used. With
> CC_USING_FENTRY, that field is ignored. Perhaps we could overload that
> field for this.

In that case, I guess we would need two versions of
ftrace_graph_ret_addr(), with the current implementation still needed
for mcount+HAVE_FUNCTION_GRAPH_FP_TEST.

Or would you want to get rid of HAVE_FUNCTION_GRAPH_FP_TEST for x86?

BTW, on a different note, should I put ftrace_graph_ret_addr() to
kernel/trace/trace_functions_graph.c so other arches can use it?

--
Josh

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