Currently running "trace.py --max-events #" (shortname "trace.py -M #")
will hang like below:

  -bash-4.2$ sudo ./trace.py -M 2 'SyS_futex'
  PID     TID     COMM            FUNC
  137727  138229  IOThreadPool0   SyS_futex
  138250  151288  OBC Dispatcher  SyS_futex
  ^C^Z
  [1]+  Stopped                 sudo ./trace.py -M 2 'SyS_futex'
  -bash-4.2$

The hang happens in perf_reader_free.

Commit cd5d4a6c ("fix a race condition between perf_reader
munmap and read") fixed a race condition by adding some coordination
between perf_reader_event_read and perf_reader_free.

In this case, however, the callback function inside perf_reader_event_read
never returns and actually calling exit(). This is because
the maximum number of perf_events have been received. The exit()
is calling BPF object cleanup() which calls perf_reader_free to
free the ring buffer. perf_reader_free got stuck since it thinks
perf_reader_event_reader did not finish yet.

To fix this, a checking for thread_id is added so that
perf_reader_free will proceed without locking if the perf_reader_read
tid is the same as perf_reader_free since this signals that
the callback function calls/triggers perf_reader_free.

After the fix,

  -bash-4.2$ sudo ./trace.py -M 2 'SyS_futex'
  PID     TID     COMM            FUNC
  137727  138178  load-monitor    SyS_futex
  6359    6440    load-monitor    SyS_futex
  -bash-4.2$
Reported-by: Teng Qin <qinteng@fb.com>
Signed-off-by: Yonghong Song <yhs@fb.com>