On Fri, 10 Nov 2023 10:51:54 +0900
Masami Hiramatsu (Google) [off-list ref] wrote:

So this patch registers ftrace_ops for each fgraph_ops to ftrace.
This means that the ftrace_graph_func() will be called twice or more
on the same function.
Thus should I call ftrace_startup() once when the first fgraph_ops
is registered? 
No, it's not enough. Actually each fgraph_ops can have different filters.
We need to define a shared filter and combine new filters to one and
use it. We also need to do it when a fgraph is unregistered.

Is there any function which makes a new filter from two (or more) filters?

So I'm guessing that we need to have a fgraph_set_filter*() operations?

When one gets added, it needs to update the ftrace_ops to include the added
functions. Or we need to have a way to create a new hash from all the
registered fgraph_ops, and have that for the ftrace_ops. Then when it gets
called, if it has more than one registered function, it needs to iterate
over the list?

-- Steve

Or, maybe we can make the common callback to find the previous ret entry on
the ret_stack and reuse it. (In this case we don't need loop on each
fgraph_array entry)