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Date:   Mon, 9 Jan 2023 19:44:11 -0800
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Subject: Re: [bpf-next v4 2/2] selftests/bpf: add test case for htab map
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From:   Martin KaFai Lau <martin.lau@linux.dev>
To:     Tonghao Zhang <tong@infragraf.org>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Hou Tao <houtao1@huawei.com>, bpf@vger.kernel.org,
        Manu Bretelle <chantra@meta.com>
References: <20230105092637.35069-1-tong@infragraf.org>
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 <6bd49922-9d38-3bf9-47e8-3208adfd2f31@linux.dev>
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On 1/9/23 7:25 PM, Martin KaFai Lau wrote:
>>>
>>> btw, from a quick look at __perf_event_overflow, I suspect doing the 
>>> bpf_map_update_elem() here instead of the fentry/perf_event_overflow above 
>>> can also reproduce the patch 1 issue?
>> No
>> bpf_overflow_handler will check the bpf_prog_active, if syscall increase it, 
>> bpf_overflow_handler will skip the bpf prog.
> 
> tbh, I am quite surprised the bpf_prog_active would be noisy enough to avoid 
> this deadlock being reproduced easily. fwiw, I just tried doing map_update here 
> and can reproduce it in the very first run.
Correcting my self. I only reproduced the warning splat but not the deadlock. 
This test is using map_update from the syscall that bumps the prog_active.

Agree that SEC("perf_event") alone won't work unless the bpf_map_update_elem() 
is not done from the syscall in prog_tests/htab_deadlock.c, eg. from another bpf 
prog.