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[3.10] gh-96387: take_gil() resets drop request before exit (GH-96869) (GH-96941) #96943

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Sep 20, 2022
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
At Python exit, sometimes a thread holding the GIL can wait forever for a
thread (usually a daemon thread) which requested to drop the GIL, whereas
the thread already exited. To fix the race condition, the thread which
requested the GIL drop now resets its request before exiting. Issue
discovered and analyzed by Mingliang ZHAO. Patch by Victor Stinner.
11 changes: 11 additions & 0 deletions Python/ceval_gil.h
Original file line number Diff line number Diff line change
Expand Up @@ -247,6 +247,7 @@ take_gil(PyThreadState *tstate)
goto _ready;
}

int drop_requested = 0;
while (_Py_atomic_load_relaxed(&gil->locked)) {
unsigned long saved_switchnum = gil->switch_number;

Expand All @@ -262,11 +263,21 @@ take_gil(PyThreadState *tstate)
{
if (tstate_must_exit(tstate)) {
MUTEX_UNLOCK(gil->mutex);
// gh-96387: If the loop requested a drop request in a previous
// iteration, reset the request. Otherwise, drop_gil() can
// block forever waiting for the thread which exited. Drop
// requests made by other threads are also reset: these threads
// may have to request again a drop request (iterate one more
// time).
if (drop_requested) {
RESET_GIL_DROP_REQUEST(interp);
}
PyThread_exit_thread();
}
assert(is_tstate_valid(tstate));

SET_GIL_DROP_REQUEST(interp);
drop_requested = 1;
}
}

Expand Down