On Fri, 2 Nov 2018 12:13:07 -0400
Steven Rostedt [off-list ref] wrote:

On Fri, 2 Nov 2018 10:43:26 -0500
Josh Poimboeuf [off-list ref] wrote:

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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);
}

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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 [off-list ref]