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Date: Thu, 12 Mar 2015 16:15:07 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Alexei Starovoitov <ast@plumgrid.com>
Cc: Ingo Molnar <mingo@kernel.org>, Steven Rostedt <rostedt@goodmis.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Jiri Olsa <jolsa@redhat.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        "David S. Miller" <davem@davemloft.net>,
        Daniel Borkmann <daniel@iogearbox.net>, linux-api@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v6 tip 2/8] tracing: attach BPF programs to kprobes
Message-ID: <20150312151507.GI2896@worktop.programming.kicks-ass.net>
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On Tue, Mar 10, 2015 at 09:18:48PM -0700, Alexei Starovoitov wrote:
> +unsigned int trace_call_bpf(struct bpf_prog *prog, void *ctx)
> +{
> +	unsigned int ret;
> +	int cpu;
> +
> +	if (in_nmi()) /* not supported yet */
> +		return 1;
> +
> +	preempt_disable_notrace();
> +
> +	cpu = raw_smp_processor_id();
> +	if (unlikely(per_cpu(bpf_prog_active, cpu)++ != 0)) {
> +		/* since some bpf program is already running on this cpu,
> +		 * don't call into another bpf program (same or different)
> +		 * and don't send kprobe event into ring-buffer,
> +		 * so return zero here
> +		 */
> +		ret = 0;
> +		goto out;
> +	}
> +
> +	rcu_read_lock();

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.

> +	ret = BPF_PROG_RUN(prog, ctx);
> +	rcu_read_unlock();
> +
> + out:
> +	per_cpu(bpf_prog_active, cpu)--;
> +	preempt_enable_notrace();
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(trace_call_bpf);