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

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Date:	Thu, 12 Mar 2015 09:43:54 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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 3/12/15 9:23 AM, Steven Rostedt wrote:
> 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.

sure. consider it done. should I respin right away or you can review
the rest?

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