Re: [PATCH bpf-next v1] selftests/bpf: Fix stdout race condition in traffic monitor

[Martin KaFai Lau <martin.lau@linux.dev>]

On 2/13/25 4:19 PM, Amery Hung wrote:
> On Thu, Feb 13, 2025 at 3:55 PM Martin KaFai Lau <martin.lau@linux.dev> wrote:
>>
>> On 2/13/25 3:32 PM, Amery Hung wrote:
>>> Fix a race condition between the main test_progs thread and the traffic
>>> monitoring thread. The traffic monitor thread tries to print a line
>>> using multiple printf and use flockfile() to prevent the line from being
>>> torn apart. Meanwhile, the main thread doing io redirection can reassign
>>> or close stdout when going through tests. A deadlock as shown below can
>>> happen.
>>>
>>>          main                      traffic_monitor_thread
>>>          ====                      ======================
>>>                                    show_transport()
>>>                                    -> flockfile(stdout)
>>>
>>> stdio_hijack_init()
>>> -> stdout = open_memstream(log_buf, log_cnt);
>>>      ...
>>>      env.subtest_state->stdout_saved = stdout;
>>>
>>>                                       ...
>>>                                       funlockfile(stdout)
>>> stdio_restore_cleanup()
>>> -> fclose(env.subtest_state->stdout_saved);
>>
>> Great debugging.
>>
>> Does it mean that the main thread will start the next test before the
>> traffic_monitor_thread has finished? Meaning the traffic_monitor_stop() does not
>> wait for the traffic_monitor_thread somehow?
> 
> That part I think is fine. The race condition here happen between
> subtests within the same "netns_new" scope. For example,
> test_flow_dissector_skb_less_direct_attach.

Got it. Thanks for the fix. Applied.