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gh-116522: Refactor _PyThreadState_DeleteExcept
(#117131)
Split `_PyThreadState_DeleteExcept` into two functions: - `_PyThreadState_RemoveExcept` removes all thread states other than one passed as an argument. It returns the removed thread states as a linked list. - `_PyThreadState_DeleteList` deletes those dead thread states. It may call destructors, so we want to "start the world" before calling `_PyThreadState_DeleteList` to avoid potential deadlocks.
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@ -218,7 +218,8 @@ extern PyThreadState * _PyThreadState_New(
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PyInterpreterState *interp,
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int whence);
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extern void _PyThreadState_Bind(PyThreadState *tstate);
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extern void _PyThreadState_DeleteExcept(PyThreadState *tstate);
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extern PyThreadState * _PyThreadState_RemoveExcept(PyThreadState *tstate);
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extern void _PyThreadState_DeleteList(PyThreadState *list);
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extern void _PyThreadState_ClearMimallocHeaps(PyThreadState *tstate);
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// Export for '_testinternalcapi' shared extension
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@ -664,6 +664,14 @@ PyOS_AfterFork_Child(void)
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goto fatal_error;
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}
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// Remove the dead thread states. We "start the world" once we are the only
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// thread state left to undo the stop the world call in `PyOS_BeforeFork`.
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// That needs to happen before `_PyThreadState_DeleteList`, because that
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// may call destructors.
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PyThreadState *list = _PyThreadState_RemoveExcept(tstate);
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_PyEval_StartTheWorldAll(&_PyRuntime);
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_PyThreadState_DeleteList(list);
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status = _PyImport_ReInitLock(tstate->interp);
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if (_PyStatus_EXCEPTION(status)) {
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goto fatal_error;
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@ -579,9 +579,8 @@ PyEval_ReleaseThread(PyThreadState *tstate)
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}
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#ifdef HAVE_FORK
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/* This function is called from PyOS_AfterFork_Child to destroy all threads
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which are not running in the child process, and clear internal locks
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which might be held by those threads. */
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/* This function is called from PyOS_AfterFork_Child to re-initialize the
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GIL and pending calls lock. */
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PyStatus
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_PyEval_ReInitThreads(PyThreadState *tstate)
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{
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@ -598,8 +597,6 @@ _PyEval_ReInitThreads(PyThreadState *tstate)
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struct _pending_calls *pending = &tstate->interp->ceval.pending;
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_PyMutex_at_fork_reinit(&pending->mutex);
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/* Destroy all threads except the current one */
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_PyThreadState_DeleteExcept(tstate);
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return _PyStatus_OK();
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}
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#endif
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@ -1934,8 +1934,11 @@ Py_FinalizeEx(void)
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will be called in the current Python thread. Since
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_PyRuntimeState_SetFinalizing() has been called, no other Python thread
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can take the GIL at this point: if they try, they will exit
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immediately. */
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_PyThreadState_DeleteExcept(tstate);
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immediately. We start the world once we are the only thread state left,
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before we call destructors. */
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PyThreadState *list = _PyThreadState_RemoveExcept(tstate);
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_PyEval_StartTheWorldAll(runtime);
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_PyThreadState_DeleteList(list);
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/* At this point no Python code should be running at all.
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The only thread state left should be the main thread of the main
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@ -1763,15 +1763,17 @@ PyThreadState_DeleteCurrent(void)
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}
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/*
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* Delete all thread states except the one passed as argument.
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* Note that, if there is a current thread state, it *must* be the one
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* passed as argument. Also, this won't touch any other interpreters
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* than the current one, since we don't know which thread state should
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* be kept in those other interpreters.
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*/
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void
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_PyThreadState_DeleteExcept(PyThreadState *tstate)
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// Unlinks and removes all thread states from `tstate->interp`, with the
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// exception of the one passed as an argument. However, it does not delete
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// these thread states. Instead, it returns the removed thread states as a
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// linked list.
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//
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// Note that if there is a current thread state, it *must* be the one
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// passed as argument. Also, this won't touch any interpreters other
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// than the current one, since we don't know which thread state should
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// be kept in those other interpreters.
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PyThreadState *
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_PyThreadState_RemoveExcept(PyThreadState *tstate)
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{
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assert(tstate != NULL);
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PyInterpreterState *interp = tstate->interp;
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@ -1783,8 +1785,7 @@ _PyThreadState_DeleteExcept(PyThreadState *tstate)
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HEAD_LOCK(runtime);
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/* Remove all thread states, except tstate, from the linked list of
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thread states. This will allow calling PyThreadState_Clear()
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without holding the lock. */
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thread states. */
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PyThreadState *list = interp->threads.head;
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if (list == tstate) {
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list = tstate->next;
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@ -1799,11 +1800,19 @@ _PyThreadState_DeleteExcept(PyThreadState *tstate)
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interp->threads.head = tstate;
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HEAD_UNLOCK(runtime);
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_PyEval_StartTheWorldAll(runtime);
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return list;
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}
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// Deletes the thread states in the linked list `list`.
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//
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// This is intended to be used in conjunction with _PyThreadState_RemoveExcept.
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void
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_PyThreadState_DeleteList(PyThreadState *list)
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{
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// The world can't be stopped because we PyThreadState_Clear() can
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// call destructors.
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assert(!_PyRuntime.stoptheworld.world_stopped);
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/* Clear and deallocate all stale thread states. Even if this
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executes Python code, we should be safe since it executes
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in the current thread, not one of the stale threads. */
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PyThreadState *p, *next;
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for (p = list; p; p = next) {
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next = p->next;
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