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GH-96071: fix deadlock in PyGILState_Ensure #96124

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Aug 19, 2022
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fix a deadlock in :c:func:`PyGILState_Ensure` when allocating new thread state. Patch by Kumar Aditya.
27 changes: 16 additions & 11 deletions Python/pystate.c
Original file line number Diff line number Diff line change
Expand Up @@ -810,7 +810,15 @@ new_threadstate(PyInterpreterState *interp)
{
PyThreadState *tstate;
_PyRuntimeState *runtime = interp->runtime;

// We don't need to allocate a thread state for the main interpreter
// (the common case), but doing it later for the other case revealed a
// reentrancy problem (deadlock). So for now we always allocate before
// taking the interpreters lock. See GH-96071.
PyThreadState *new_tstate = alloc_threadstate();
int used_newtstate;
if (new_tstate == NULL) {
return NULL;
}
/* We serialize concurrent creation to protect global state. */
HEAD_LOCK(runtime);

Expand All @@ -822,18 +830,15 @@ new_threadstate(PyInterpreterState *interp)
if (old_head == NULL) {
// It's the interpreter's initial thread state.
assert(id == 1);

used_newtstate = 0;
tstate = &interp->_initial_thread;
}
else {
// Every valid interpreter must have at least one thread.
assert(id > 1);
assert(old_head->prev == NULL);

tstate = alloc_threadstate();
if (tstate == NULL) {
goto error;
}
used_newtstate = 1;
tstate = new_tstate;
// Set to _PyThreadState_INIT.
memcpy(tstate,
&initial._main_interpreter._initial_thread,
Expand All @@ -844,11 +849,11 @@ new_threadstate(PyInterpreterState *interp)
init_threadstate(tstate, interp, id, old_head);

HEAD_UNLOCK(runtime);
if (!used_newtstate) {
// Must be called with lock unlocked to avoid re-entrancy deadlock.
PyMem_RawFree(new_tstate);
}
return tstate;

error:
HEAD_UNLOCK(runtime);
return NULL;
}

PyThreadState *
Expand Down