Re: [RFC PATCH v2 12/31] function_graph: Have the instances use their own ftrace_ops for filtering

Masami Hiramatsu
2023-11-10T01:52:05Z

Hi,

I found strange behavior that if we set two or more probes on the same function,
its callback called twice or more. Also, the stacktrace failed.
And I found the root cause is here;

On Wed, 8 Nov 2023 23:26:42 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> @@ -910,9 +917,10 @@ int register_ftrace_graph(struct fgraph_ops *gops)
> */
> ftrace_graph_return = return_run;
> ftrace_graph_entry = entry_run;
> -
> - ret = ftrace_startup(&graph_ops, FTRACE_START_FUNC_RET);
> + command = FTRACE_START_FUNC_RET;
> }
> +
> + ret = ftrace_startup(&gops->ops, command);

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?

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)

Thank you,

--
Masami Hiramatsu (Google) <mhiramat@kernel.org>