From: Alexei Starovoitov <ast@fb.com>
Subject: Re: [PATCH net-next 2/3] bpf: introduce BPF_MAP_TYPE_STACK_TRACE
Date: Thu, 25 Feb 2016 08:42:40 -0800
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Cc: "David S. Miller" <davem@davemloft.net>,
	Ingo Molnar <mingo@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Wang Nan <wangnan0@huawei.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Brendan Gregg <brendan.d.gregg@gmail.com>,
	<netdev@vger.kernel.org>, <linux-kernel@vger.kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
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On 2/25/16 6:23 AM, Peter Zijlstra wrote:
>> +	id = hash & (smap->n_buckets - 1);
> Its not at all clear where the corresponding rcu_read_lock() is at.
>
>> >+	bucket = rcu_dereference(smap->buckets[id]);

bpf programs of all types are always executing under rcu_read_lock().
This is fundamental for maps and majority of the helpers
that's why there is no WARN_ON(rcu_read_lock_held) in this helper,
since we already have it in many other places.
The rcu_read_lock() for kprobe type is in trace_call_bpf().