linux-kernel - Re: [PATCH v20 00/19] tracing: fprobe: function_graph: Multi-function graph and fprobe on fgraph

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Message-ID: <20241207094710.21cdaceb@rorschach.local.home>
Date: Sat, 7 Dec 2024 09:47:10 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Florent Revest
 <revest@...omium.org>, linux-trace-kernel@...r.kernel.org, LKML
 <linux-kernel@...r.kernel.org>, Martin KaFai Lau <martin.lau@...ux.dev>,
 bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>, Jiri Olsa
 <jolsa@...nel.org>, Alan Maguire <alan.maguire@...cle.com>, Mark Rutland
 <mark.rutland@....com>, linux-arch@...r.kernel.org
Subject: Re: [PATCH v20 00/19] tracing: fprobe: function_graph:
 Multi-function graph and fprobe on fgraph

On Sat, 7 Dec 2024 15:51:36 +0900
Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:

> This checks the handler is called with preempt off.
> 
> On x86_64, the ftrace_graph_func calls function_graph_enter_regs() with
> ftrace_test_recursion_trylock() as below;
> 
> void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
> 		       struct ftrace_ops *op, struct ftrace_regs *fregs)
> {
> 	struct pt_regs *regs = &arch_ftrace_regs(fregs)->regs;
> 	unsigned long *stack = (unsigned long *)kernel_stack_pointer(regs);
> 	unsigned long return_hooker = (unsigned long)&return_to_handler;
> 	unsigned long *parent = (unsigned long *)stack;
> 	int bit;
> 
> 	if (unlikely(skip_ftrace_return()))
> 		return;
> 
> 	bit = ftrace_test_recursion_trylock(ip, *parent);
> 	if (bit < 0)
> 		return;
> 
> 	if (!function_graph_enter_regs(*parent, ip, 0, parent, fregs))
> 		*parent = return_hooker;
> 
> 	ftrace_test_recursion_unlock(bit);
> }
> 
> However, arm64 version does not;

Hmm, I think we can move that recursion check out of the arch/x86 code
and into ftrace_graph_enter_regs().

-- Steve