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gh-116522: Stop the world before fork() and during shutdown (#116607)
This changes the free-threaded build to perform a stop-the-world pause before deleting other thread states when forking and during shutdown. This fixes some crashes when using multiprocessing and during shutdown when running with `PYTHON_GIL=0`. This also changes `PyOS_BeforeFork` to acquire the runtime lock (i.e., `HEAD_LOCK(&_PyRuntime)`) before forking to ensure that data protected by the runtime lock (and not just the GIL or stop-the-world) is in a consistent state before forking.
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@ -613,11 +613,16 @@ PyOS_BeforeFork(void)
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run_at_forkers(interp->before_forkers, 1);
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_PyImport_AcquireLock(interp);
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_PyEval_StopTheWorldAll(&_PyRuntime);
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HEAD_LOCK(&_PyRuntime);
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}
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void
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PyOS_AfterFork_Parent(void)
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{
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HEAD_UNLOCK(&_PyRuntime);
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_PyEval_StartTheWorldAll(&_PyRuntime);
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PyInterpreterState *interp = _PyInterpreterState_GET();
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if (_PyImport_ReleaseLock(interp) <= 0) {
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Py_FatalError("failed releasing import lock after fork");
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@ -632,6 +637,7 @@ PyOS_AfterFork_Child(void)
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PyStatus status;
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_PyRuntimeState *runtime = &_PyRuntime;
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// re-creates runtime->interpreters.mutex (HEAD_UNLOCK)
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status = _PyRuntimeState_ReInitThreads(runtime);
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if (_PyStatus_EXCEPTION(status)) {
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goto fatal_error;
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@ -7731,10 +7737,15 @@ os_register_at_fork_impl(PyObject *module, PyObject *before,
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// running in the process. Best effort, silent if unable to count threads.
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// Constraint: Quick. Never overcounts. Never leaves an error set.
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//
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// This code might do an import, thus acquiring the import lock, which
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// PyOS_BeforeFork() also does. As this should only be called from
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// the parent process, it is in the same thread so that works.
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static void warn_about_fork_with_threads(const char* name) {
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// This should only be called from the parent process after
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// PyOS_AfterFork_Parent().
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static void
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warn_about_fork_with_threads(const char* name)
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{
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// It's not safe to issue the warning while the world is stopped, because
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// other threads might be holding locks that we need, which would deadlock.
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assert(!_PyRuntime.stoptheworld.world_stopped);
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// TODO: Consider making an `os` module API to return the current number
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// of threads in the process. That'd presumably use this platform code but
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// raise an error rather than using the inaccurate fallback.
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@ -7858,9 +7869,10 @@ os_fork1_impl(PyObject *module)
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/* child: this clobbers and resets the import lock. */
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PyOS_AfterFork_Child();
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} else {
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warn_about_fork_with_threads("fork1");
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/* parent: release the import lock. */
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PyOS_AfterFork_Parent();
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// After PyOS_AfterFork_Parent() starts the world to avoid deadlock.
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warn_about_fork_with_threads("fork1");
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}
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if (pid == -1) {
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errno = saved_errno;
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@ -7906,9 +7918,10 @@ os_fork_impl(PyObject *module)
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/* child: this clobbers and resets the import lock. */
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PyOS_AfterFork_Child();
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} else {
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warn_about_fork_with_threads("fork");
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/* parent: release the import lock. */
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PyOS_AfterFork_Parent();
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// After PyOS_AfterFork_Parent() starts the world to avoid deadlock.
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warn_about_fork_with_threads("fork");
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}
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if (pid == -1) {
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errno = saved_errno;
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@ -8737,9 +8750,10 @@ os_forkpty_impl(PyObject *module)
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/* child: this clobbers and resets the import lock. */
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PyOS_AfterFork_Child();
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} else {
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warn_about_fork_with_threads("forkpty");
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/* parent: release the import lock. */
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PyOS_AfterFork_Parent();
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// After PyOS_AfterFork_Parent() starts the world to avoid deadlock.
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warn_about_fork_with_threads("forkpty");
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}
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if (pid == -1) {
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return posix_error();
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@ -1911,6 +1911,9 @@ Py_FinalizeEx(void)
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int malloc_stats = tstate->interp->config.malloc_stats;
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#endif
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/* Ensure that remaining threads are detached */
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_PyEval_StopTheWorldAll(runtime);
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/* Remaining daemon threads will automatically exit
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when they attempt to take the GIL (ex: PyEval_RestoreThread()). */
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_PyInterpreterState_SetFinalizing(tstate->interp, tstate);
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@ -1692,6 +1692,10 @@ _PyThreadState_DeleteExcept(PyThreadState *tstate)
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PyInterpreterState *interp = tstate->interp;
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_PyRuntimeState *runtime = interp->runtime;
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#ifdef Py_GIL_DISABLED
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assert(runtime->stoptheworld.world_stopped);
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#endif
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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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@ -1710,6 +1714,8 @@ _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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/* 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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