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Date: Sat, 25 May 2024 18:44:14 +0900
From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>, 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>, Jiri Olsa <jolsa@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 v10 07/36] function_graph: Allow multiple users to
 attach to function graph
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On Fri, 24 May 2024 21:32:08 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Tue,  7 May 2024 23:09:22 +0900
> "Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:
> 
> > @@ -109,6 +244,21 @@ ftrace_push_return_trace(unsigned long ret, unsigned long func,
> >  	if (!current->ret_stack)
> >  		return -EBUSY;
> >  
> > +	/*
> > +	 * At first, check whether the previous fgraph callback is pushed by
> > +	 * the fgraph on the same function entry.
> > +	 * But if @func is the self tail-call function, we also need to ensure
> > +	 * the ret_stack is not for the previous call by checking whether the
> > +	 * bit of @fgraph_idx is set or not.
> > +	 */
> > +	ret_stack = get_ret_stack(current, current->curr_ret_stack, &offset);
> > +	if (ret_stack && ret_stack->func == func &&
> > +	    get_fgraph_type(current, offset + FGRAPH_FRAME_OFFSET) == FGRAPH_TYPE_BITMAP &&
> > +	    !is_fgraph_index_set(current, offset + FGRAPH_FRAME_OFFSET, fgraph_idx))
> > +		return offset + FGRAPH_FRAME_OFFSET;
> > +
> > +	val = (FGRAPH_TYPE_RESERVED << FGRAPH_TYPE_SHIFT) | FGRAPH_FRAME_OFFSET;
> > +
> >  	BUILD_BUG_ON(SHADOW_STACK_SIZE % sizeof(long));
> 
> I'm trying to figure out what the above is trying to do. This gets called
> once in function_graph_enter() (or function_graph_enter_ops()). What
> exactly are you trying to catch here?

Aah, good catch! This was originally for catching the self tail-call case with
multiple fgraph callback on the same function, but it was my misread.
In later patch ([12/36]), we introduced function_graph_enter_ops() so that
we can skip checking hash table and directly pass the fgraph_ops to user
callback. I thought this function_graph_enter_ops() is used even if multiple
fgraph is set on the same function. In this case, we always need to check the
stack can be reused(pushed by other fgraph_ops on the same function) or not.
But as we discussed, the function_graph_enter_ops() is used only when only
one fgraph is set on the function (if there are multiple fgraphs are set on
the same function, use function_graph_enter() ), we are sure that 
ftrace_push_return_trace() is called only once on hooking the function entry.
Thus we don't need to reuse it.

> 
> Is it from this email:
> 
>   https://lore.kernel.org/all/20231110105154.df937bf9f200a0c16806c522@kernel.org/
> 
> As that's the last version before you added the above code.
> 
> But you also noticed it may not be needed, but triggered a crash without it
> in v3:
> 
>   https://lore.kernel.org/all/20231205234511.3839128259dfec153ea7da81@kernel.org/
> 
> I removed this code in my version and it runs just fine. Perhaps there was
> another bug that this was hiding that you fixed in later versions?

No problem. I think we can remove this block safely.

Thank you,

> 
> -- Steve
> 


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