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Date:   Sat, 3 Nov 2018 22:00:12 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
        "David S. Miller" <davem@davemloft.net>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
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        Namhyung Kim <namhyung@kernel.org>,
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Subject: Re: [PATCH v3 1/2] kretprobe: produce sane stack traces
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On Fri, 2 Nov 2018 12:13:07 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Fri, 2 Nov 2018 10:43:26 -0500
> Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> 
> > > I'll hopefully have a prototype ready by plumbers.  
> > 
> > Why do we need multiple users?  It would be a lot simpler if we could
> > just enforce a single user per fgraphed/kretprobed function (and return
> > -EBUSY if it's already being traced/probed).
> 
> Because that means if function graph tracer is active, then you can't
> do a kretprobe, and vice versa. I'd really like to have it working for
> multiple users, then we could trace different graph functions and store
> them in different buffers. It would also allow for perf to use function
> graph tracer too.

Steve, how woul you allow multiple users on it? Something like this?

ret_trampoline_multiple(){
   list_for_each(handler, &shadow_entry[i].handlers, list)
	handler(shadow_entry[i]);
   restore_retval_and_jump_to(shadow_entry[i].orig);
}


> > > And this too will require each architecture to probably change. As a
> > > side project to this, I'm going to try to consolidate the function
> > > graph code among all the architectures as well. Not an easy task.  
> > 
> > Do you mean implementing HAVE_FUNCTION_GRAPH_RET_ADDR_PTR for all the
> > arches?  If so, I think have an old crusty patch which attempted to
> > that.  I could try to dig it up if you're interested.
> > 
> 
> I'd like to have that, but it still requires some work. But I'd just
> the truly architecture dependent code be in the architecture (basically
> the asm code), and have the ability to move most of the duplicate code
> out of the archs.

I will also do that for kretprobe handlers.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>