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From: Jiri Olsa <olsajiri@gmail.com>
Date: Tue, 19 Dec 2023 14:23:37 +0100
To: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Florent Revest <revest@chromium.org>,
	linux-trace-kernel@vger.kernel.org,
	LKML <linux-kernel@vger.kernel.org>,
	Martin KaFai Lau <martin.lau@linux.dev>, bpf <bpf@vger.kernel.org>,
	Sven Schnelle <svens@linux.ibm.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Alan Maguire <alan.maguire@oracle.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>, Guo Ren <guoren@kernel.org>
Subject: Re: [PATCH v5 06/34] function_graph: Allow multiple users to attach
 to function graph
Message-ID: <ZYGZWWqwtSP82Sja@krava>
References: <170290509018.220107.1347127510564358608.stgit@devnote2>
 <170290516454.220107.14775763404510245361.stgit@devnote2>
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In-Reply-To: <170290516454.220107.14775763404510245361.stgit@devnote2>

On Mon, Dec 18, 2023 at 10:12:45PM +0900, Masami Hiramatsu (Google) wrote:

SNIP

>  /* Both enabled by default (can be cleared by function_graph tracer flags */
>  static bool fgraph_sleep_time = true;
>  
> @@ -126,9 +247,34 @@ ftrace_push_return_trace(unsigned long ret, unsigned long func,
>  	calltime = trace_clock_local();
>  
>  	index = current->curr_ret_stack;
> -	RET_STACK_INC(current->curr_ret_stack);
> +	/* ret offset = 1 ; type = reserved */
> +	current->ret_stack[index + FGRAPH_RET_INDEX] = 1;
>  	ret_stack = RET_STACK(current, index);
> +	ret_stack->ret = ret;
> +	/*
> +	 * The unwinders expect curr_ret_stack to point to either zero
> +	 * or an index where to find the next ret_stack. Even though the
> +	 * ret stack might be bogus, we want to write the ret and the
> +	 * index to find the ret_stack before we increment the stack point.
> +	 * If an interrupt comes in now before we increment the curr_ret_stack
> +	 * it may blow away what we wrote. But that's fine, because the
> +	 * index will still be correct (even though the 'ret' won't be).
> +	 * What we worry about is the index being correct after we increment
> +	 * the curr_ret_stack and before we update that index, as if an
> +	 * interrupt comes in and does an unwind stack dump, it will need
> +	 * at least a correct index!
> +	 */
>  	barrier();
> +	current->curr_ret_stack += FGRAPH_RET_INDEX + 1;
> +	/*
> +	 * This next barrier is to ensure that an interrupt coming in
> +	 * will not corrupt what we are about to write.
> +	 */
> +	barrier();
> +
> +	/* Still keep it reserved even if an interrupt came in */
> +	current->ret_stack[index + FGRAPH_RET_INDEX] = 1;

seems like this was set already few lines above?

jirka

> +
>  	ret_stack->ret = ret;
>  	ret_stack->func = func;
>  	ret_stack->calltime = calltime;
> @@ -159,6 +305,12 @@ int function_graph_enter(unsigned long ret, unsigned long func,
>  			 unsigned long frame_pointer, unsigned long *retp)
>  {
>  	struct ftrace_graph_ent trace;
> +	int offset;
> +	int start;
> +	int type;
> +	int val;
> +	int cnt = 0;
> +	int i;
>  
>  #ifndef CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS
>  	/*

SNIP