From mboxrd@z Thu Jan  1 00:00:00 1970
From: Alexei Starovoitov <ast@fb.com>
Subject: Re: [RFC PATCH bpf-next 2/6] bpf: add bpf_get_stack helper
Date: Sun, 8 Apr 2018 22:02:38 -0700
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To: Yonghong Song <yhs@fb.com>, <daniel@iogearbox.net>,
        <netdev@vger.kernel.org>
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On 4/8/18 9:53 PM, Yonghong Song wrote:
>>> @@ -1004,7 +1007,8 @@ static void __bpf_prog_put(struct bpf_prog
>>> *prog, bool do_idr_lock)
>>>              bpf_prog_kallsyms_del(prog->aux->func[i]);
>>>          bpf_prog_kallsyms_del(prog);
>>>
>>> -        call_rcu(&prog->aux->rcu, __bpf_prog_put_rcu);
>>> +        synchronize_rcu();
>>> +        __bpf_prog_put_rcu(&prog->aux->rcu);
>>
>> there should have been lockdep splat.
>> We cannot call synchronize_rcu here, since we cannot sleep
>> in some cases.
>
> Let me double check this. The following is the reason
> why I am using synchronize_rcu().
>
> With call_rcu(&prog->aux->rcu, __bpf_prog_put_rcu) and
> _bpf_prog_put_rcu calls put_callchain_buffers() which
> calls mutex_lock(), the runtime with CONFIG_DEBUG_ATOMIC_SLEEP=y
> will complains since potential sleep inside the call_rcu is not
> allowed.

I see. Indeed. We cannot call put_callchain_buffers() from rcu callback,
but doing synchronize_rcu() here is also not possible.
How about moving put_callchain into bpf_prog_free_deferred() ?