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gh-119369: Fix deadlock during thread exit in free-threaded build #119528

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Merged
colesbury merged 1 commit into from
May 31, 2024
Merged

gh-119369: Fix deadlock during thread exit in free-threaded build #119528

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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2024-05-24-21-16-52.gh-issue-119369.qBThho.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix deadlock during thread deletion in free-threaded build, which could
occur when the GIL was enabled at runtime.
21 changes: 12 additions & 9 deletions Python/pystate.c
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Expand Up @@ -1751,7 +1751,7 @@ decrement_stoptheworld_countdown(struct _stoptheworld_state *stw);

/* Common code for PyThreadState_Delete() and PyThreadState_DeleteCurrent() */
static void
tstate_delete_common(PyThreadState *tstate)
tstate_delete_common(PyThreadState *tstate, int release_gil)
{
assert(tstate->_status.cleared && !tstate->_status.finalized);
tstate_verify_not_active(tstate);
Expand Down Expand Up @@ -1793,10 +1793,6 @@ tstate_delete_common(PyThreadState *tstate)

HEAD_UNLOCK(runtime);

#ifdef Py_GIL_DISABLED
_Py_qsbr_unregister(tstate);
#endif

// XXX Unbind in PyThreadState_Clear(), or earlier
// (and assert not-equal here)?
if (tstate->_status.bound_gilstate) {
Expand All @@ -1807,6 +1803,14 @@ tstate_delete_common(PyThreadState *tstate)
// XXX Move to PyThreadState_Clear()?
clear_datastack(tstate);

if (release_gil) {
_PyEval_ReleaseLock(tstate->interp, tstate, 1);
}

#ifdef Py_GIL_DISABLED
_Py_qsbr_unregister(tstate);
#endif

tstate->_status.finalized = 1;
}

Expand All @@ -1818,7 +1822,7 @@ zapthreads(PyInterpreterState *interp)
when the threads are all really dead (XXX famous last words). */
while ((tstate = interp->threads.head) != NULL) {
tstate_verify_not_active(tstate);
tstate_delete_common(tstate);
tstate_delete_common(tstate, 0);
free_threadstate((_PyThreadStateImpl *)tstate);
}
}
Expand All @@ -1829,7 +1833,7 @@ PyThreadState_Delete(PyThreadState *tstate)
{
_Py_EnsureTstateNotNULL(tstate);
tstate_verify_not_active(tstate);
tstate_delete_common(tstate);
tstate_delete_common(tstate, 0);
free_threadstate((_PyThreadStateImpl *)tstate);
}

Expand All @@ -1842,8 +1846,7 @@ _PyThreadState_DeleteCurrent(PyThreadState *tstate)
_Py_qsbr_detach(((_PyThreadStateImpl *)tstate)->qsbr);
#endif
current_fast_clear(tstate->interp->runtime);
tstate_delete_common(tstate);
_PyEval_ReleaseLock(tstate->interp, tstate, 1);
tstate_delete_common(tstate, 1); // release GIL as part of call
free_threadstate((_PyThreadStateImpl *)tstate);
}

Expand Down
5 changes: 5 additions & 0 deletions Python/qsbr.c
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Expand Up @@ -236,6 +236,11 @@ _Py_qsbr_unregister(PyThreadState *tstate)
struct _qsbr_shared *shared = &tstate->interp->qsbr;
struct _PyThreadStateImpl *tstate_imp = (_PyThreadStateImpl*) tstate;

// gh-119369: GIL must be released (if held) to prevent deadlocks, because
// we might not have an active tstate, which means taht blocking on PyMutex
// locks will not implicitly release the GIL.
assert(!tstate->_status.holds_gil);

PyMutex_Lock(&shared->mutex);
// NOTE: we must load (or reload) the thread state's qbsr inside the mutex
// because the array may have been resized (changing tstate->qsbr) while
Expand Down