The setting and checking of 'done' contains a rare race where the signal
handler setting 'done' is run after checking to break the loop, but
before waiting in evlist__poll(). In this case, the main loop won't wake
up until either another signal is sent, or the perf data fd causes a
wake up.

The following simple script can trigger this condition (but you might
need to run it for several hours):

for ((i = 0; i >= 0; i++)) ; do
  echo "Loop $i"
  delay=$(echo "scale=4; 0.1 * $RANDOM/32768" | bc)
  ./perf record -- sleep 30000000 >/dev/null&
  pid=$!
  sleep $delay
  kill -TERM $pid
  echo "PID $pid"
  wait $pid
done

At some point, the loop will stall. Adding logging, even though perf has
received the SIGTERM and set 'done = 1', perf will remain sleeping until
a second signal is sent.

Committer notes:

Make this dependent on HAVE_EVENTFD_SUPPORT, so that we continue
building on older systems without the eventfd syscall.
Signed-off-by: Anand K Mistry <amistry@google.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200513122012.v3.1.I4d7421c6bbb1f83ea58419082481082e19097841@changeidSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>