From c86088c30ed3c13894b447e1f47feecc6f67c870 Mon Sep 17 00:00:00 2001 From: Victor Stinner Date: Fri, 16 Sep 2022 16:48:13 +0200 Subject: [PATCH] gh-96387: take_gil() resets drop request before exit 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. take_gil() now calls RESET_GIL_DROP_REQUEST() before PyThread_exit_thread() if it called SET_GIL_DROP_REQUEST to fix a race condition with drop_gil(). Issue discovered and analyzed by Mingliang ZHAO. --- .../2022-09-16-16-54-35.gh-issue-96387.GRzewg.rst | 5 +++++ Python/ceval_gil.c | 11 +++++++++++ 2 files changed, 16 insertions(+) create mode 100644 Misc/NEWS.d/next/Core and Builtins/2022-09-16-16-54-35.gh-issue-96387.GRzewg.rst diff --git a/Misc/NEWS.d/next/Core and Builtins/2022-09-16-16-54-35.gh-issue-96387.GRzewg.rst b/Misc/NEWS.d/next/Core and Builtins/2022-09-16-16-54-35.gh-issue-96387.GRzewg.rst new file mode 100644 index 00000000000000..611ab94bc636a6 --- /dev/null +++ b/Misc/NEWS.d/next/Core and Builtins/2022-09-16-16-54-35.gh-issue-96387.GRzewg.rst @@ -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. diff --git a/Python/ceval_gil.c b/Python/ceval_gil.c index a6790866766795..fd737b5738e889 100644 --- a/Python/ceval_gil.c +++ b/Python/ceval_gil.c @@ -369,6 +369,7 @@ take_gil(PyThreadState *tstate) goto _ready; } + int drop_requested = 0; while (_Py_atomic_load_relaxed(&gil->locked)) { unsigned long saved_switchnum = gil->switch_number; @@ -384,11 +385,21 @@ 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; } }