netdev - Re: [PATCH v6 tip 2/8] tracing: attach BPF programs to kprobes

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Message-ID: <20150312122345.0b28c266@gandalf.local.home>
Date:	Thu, 12 Mar 2015 12:23:45 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <daniel@...earbox.net>,
	linux-api@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 tip 2/8] tracing: attach BPF programs to kprobes

On Thu, 12 Mar 2015 09:18:34 -0700
Alexei Starovoitov <ast@...mgrid.com> wrote:

> > You've so far tried very hard to not get into tracing; and then you call
> > rcu_read_lock() :-)
> >
> > So either document why this isn't a problem, provide
> > rcu_read_lock_notrace() or switch to RCU-sched and thereby avoid the
> > problem.
> 
> I don't see the problem.
> I actually do turn on func and func_graph tracers from time to time to
> debug bpf core itself. Why would tracing interfere with anything that
> this patch is doing? When we're inside tracing processing, we need to
> use only _notrace() helpers otherwise recursion will hurt, but this
> code is not invoked from there. It's called from 
> kprobe_ftrace_handler|kprobe_int3_handler->kprobe_dispatcher->
> kprobe_perf_func->trace_call_bpf which all are perfectly traceable.
> Probably my copy paste of preempt_disable_notrace() line from
> stack_trace_call() became source of confusion? I believe
> normal preempt_disable() here will be just fine.
> It's actually redundant too, since preemption is disabled by kprobe
> anyway. Please help me understand what I'm missing.

As Peter stated, "You've so far tried very hard to not get into
tracing", which the preempt_disable_notrace() is the source of confusion.

Just remove the _notrace() part, as it doesn't make sense to have part
not traced, and other parts traced for no apparent reason.

-- Steve
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