linux-kernel - Re: [RFC PATCH v2 26/31] fprobe: Rewrite fprobe on function-graph tracer

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Date:   Sat, 11 Nov 2023 12:01:00 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.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>, Sven Schnelle <svens@...ux.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alan Maguire <alan.maguire@...cle.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Guo Ren <guoren@...nel.org>
Subject: Re: [RFC PATCH v2 26/31] fprobe: Rewrite fprobe on function-graph
 tracer

On Fri, 10 Nov 2023 20:44:22 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 10 Nov 2023 16:17:39 +0900
> Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
> 
> > > +	used = 0;
> > > +	hlist_for_each_entry_from_rcu(node, hlist) {
> > > +		if (node->addr != func)
> > > +			break;
> > > +		fp = READ_ONCE(node->fp);
> > > +		if (!fp || fprobe_disabled(fp))
> > > +			continue;
> > > +
> > > +		if (fprobe_shared_with_kprobes(fp))
> > > +			ret = __fprobe_kprobe_handler(func, ret_ip,
> > > +					fp, fregs, fgraph_data + used);
> > > +		else
> > > +			ret = __fprobe_handler(func, ret_ip, fp,
> > > +					fregs, fgraph_data + used);  
> > 
> > 
> > Since the fgraph callback is under rcu-locked but not preempt-disabled,
> 
> rcu-locked? The only rcu-locked is task rcu.

Hmm, it might be my misread. But I don't get any warning from
find_first_fprobe_node(), which uses hlist_for_each_entry_rcu()
so isn't it rcu locked?

> 
> > fprobe unittest fails. I need to add preempt_disable_notrace() and
> > preempt_enable_notrace() around this. Note that kprobe_busy_begin()/end()
> > also access to per-cpu variable, so it requires to disable preemption.
> 
> 
> Just around the __fprobe_*handler()? Or the loop?

Just around the __fprobe*handler().

Thank you,


> 
> -- Steve


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