Store holds_gil flag on PyThreadState instead, and use it where appro…

…priate
python · colesbury · May 23, 2024 · May 7, 2024 · May 8, 2024 · May 8, 2024
commit b91a1d59862b1a95d098574347d5b45d62265f87
@@ -83,14 +83,20 @@ struct _ts {
         unsigned int bound_gilstate:1;
         /* Currently in use (maybe holds the GIL). */
         unsigned int active:1;
+#ifdef Py_GIL_DISABLED
+        /* Currently holds the GIL. */
+        unsigned int holds_gil:1;
+#else
+        unsigned int _unused:1;
+#endif
 
         /* various stages of finalization */
         unsigned int finalizing:1;
         unsigned int cleared:1;
         unsigned int finalized:1;
 
         /* padding to align to 4 bytes */
-        unsigned int :24;
+        unsigned int :23;
     } _status;
 #ifdef Py_BUILD_CORE
 #  define _PyThreadState_WHENCE_NOTSET -1

diff --git a/Include/internal/pycore_ceval.h b/Include/internal/pycore_ceval.h
@@ -131,11 +131,10 @@ extern int _PyEval_ThreadsInitialized(void);
 extern void _PyEval_InitGIL(PyThreadState *tstate, int own_gil);
 extern void _PyEval_FiniGIL(PyInterpreterState *interp);
 
-// Acquire the GIL and return 1. In free-threaded builds, this function may
-// return 0 to indicate that the GIL was disabled and therefore not acquired.
-extern int _PyEval_AcquireLock(PyThreadState *tstate);
+extern void _PyEval_AcquireLock(PyThreadState *tstate);
 
-extern void _PyEval_ReleaseLock(PyInterpreterState *, PyThreadState *);
+extern void _PyEval_ReleaseLock(PyInterpreterState *, PyThreadState *,
+                                int thread_dying);
 
 #ifdef Py_GIL_DISABLED
 // Returns 0 or 1 if the GIL for the given thread's interpreter is disabled or

@@ -205,59 +205,60 @@ static void recreate_gil(struct _gil_runtime_state *gil)
 }
 #endif
 
-static void
-drop_gil_impl(struct _gil_runtime_state *gil)
+static inline void
+drop_gil_impl(PyThreadState *tstate, struct _gil_runtime_state *gil)
 {
     MUTEX_LOCK(gil->mutex);
     _Py_ANNOTATE_RWLOCK_RELEASED(&gil->locked, /*is_write=*/1);
     _Py_atomic_store_int_relaxed(&gil->locked, 0);
+#ifdef Py_GIL_DISABLED
+    tstate->_status.holds_gil = 0;
+#endif
     COND_SIGNAL(gil->cond);
     MUTEX_UNLOCK(gil->mutex);
 }
 
 static void
-drop_gil(PyInterpreterState *interp, PyThreadState *tstate)
+drop_gil(PyInterpreterState *interp, PyThreadState *tstate, int thread_dying)
 {
     struct _ceval_state *ceval = &interp->ceval;
-    /* If tstate is NULL, the caller is indicating that we're releasing
+    /* If thread_dying is true, the caller is indicating that we're releasing
        the GIL for the last time in this thread.  This is particularly
        relevant when the current thread state is finalizing or its
        interpreter is finalizing (either may be in an inconsistent
        state).  In that case the current thread will definitely
        never try to acquire the GIL again. */
     // XXX It may be more correct to check tstate->_status.finalizing.
-    // XXX assert(tstate == NULL || !tstate->_status.cleared);
+    // XXX assert(thread_dying || !tstate->_status.cleared);
 
+    assert(tstate != NULL);
     struct _gil_runtime_state *gil = ceval->gil;
 #ifdef Py_GIL_DISABLED
-    if (!_Py_atomic_load_int_relaxed(&gil->enabled)) {
+    if (!tstate->_status.holds_gil) {
         return;
     }
 #endif
     if (!_Py_atomic_load_int_relaxed(&gil->locked)) {
         Py_FatalError("drop_gil: GIL is not locked");
     }
 
-    /* tstate is allowed to be NULL (early interpreter init) */
-    if (tstate != NULL) {
+    if (!thread_dying) {
         /* Sub-interpreter support: threads might have been switched
            under our feet using PyThreadState_Swap(). Fix the GIL last
            holder variable so that our heuristics work. */
         _Py_atomic_store_ptr_relaxed(&gil->last_holder, tstate);
     }
 
-    drop_gil_impl(gil);
+    drop_gil_impl(tstate, gil);
 
 #ifdef FORCE_SWITCHING
-    /* We check tstate first in case we might be releasing the GIL for
-       the last time in this thread.  In that case there's a possible
-       race with tstate->interp getting deleted after gil->mutex is
-       unlocked and before the following code runs, leading to a crash.
-       We can use (tstate == NULL) to indicate the thread is done with
-       the GIL, and that's the only time we might delete the
-       interpreter, so checking tstate first prevents the crash.
-       See https://github.com/python/cpython/issues/104341. */
-    if (tstate != NULL &&
+    /* We might be releasing the GIL for the last time in this thread.  In that
+       case there's a possible race with tstate->interp getting deleted after
+       gil->mutex is unlocked and before the following code runs, leading to a
+       crash.  We can use thread_dying to indicate the thread is done with the
+       GIL, and that's the only time we might delete the interpreter.  See
+       https://github.com/python/cpython/issues/104341. */
+    if (!thread_dying &&
         _Py_eval_breaker_bit_is_set(tstate, _PY_GIL_DROP_REQUEST_BIT)) {
         MUTEX_LOCK(gil->switch_mutex);
         /* Not switched yet => wait */
@@ -284,7 +285,7 @@ drop_gil(PyInterpreterState *interp, PyThreadState *tstate)
    tstate must be non-NULL.
 
    Returns 1 if the GIL was acquired, or 0 if not. */
-static int
+static void
 take_gil(PyThreadState *tstate)
 {
     int err = errno;
@@ -309,7 +310,7 @@ take_gil(PyThreadState *tstate)
     struct _gil_runtime_state *gil = interp->ceval.gil;
 #ifdef Py_GIL_DISABLED
     if (!_Py_atomic_load_int_relaxed(&gil->enabled)) {
-        return 0;
+        return;
     }
 #endif
 
@@ -358,10 +359,10 @@ take_gil(PyThreadState *tstate)
     if (!_Py_atomic_load_int_relaxed(&gil->enabled)) {
         // Another thread disabled the GIL between our check above and
         // now. Don't take the GIL, signal any other waiting threads, and
-        // return 0.
+        // return.
         COND_SIGNAL(gil->cond);
         MUTEX_UNLOCK(gil->mutex);
-        return 0;
+        return;
     }
 #endif
 
@@ -371,6 +372,9 @@ take_gil(PyThreadState *tstate)
     MUTEX_LOCK(gil->switch_mutex);
 #endif
     /* We now hold the GIL */
+#ifdef Py_GIL_DISABLED
+    tstate->_status.holds_gil = 1;
+#endif
     _Py_atomic_store_int_relaxed(&gil->locked, 1);
     _Py_ANNOTATE_RWLOCK_ACQUIRED(&gil->locked, /*is_write=*/1);
 
@@ -393,9 +397,7 @@ take_gil(PyThreadState *tstate)
            in take_gil() while the main thread called
            wait_for_thread_shutdown() from Py_Finalize(). */
         MUTEX_UNLOCK(gil->mutex);
-        /* Passing NULL to drop_gil() indicates that this thread is about to
-           terminate and will never hold the GIL again. */
-        drop_gil(interp, NULL);
+        drop_gil(interp, tstate, 1);
         PyThread_exit_thread();
     }
     assert(_PyThreadState_CheckConsistency(tstate));
@@ -406,7 +408,7 @@ take_gil(PyThreadState *tstate)
     MUTEX_UNLOCK(gil->mutex);
 
     errno = err;
-    return 1;
+    return;
 }
 
 void _PyEval_SetSwitchInterval(unsigned long microseconds)
@@ -452,9 +454,16 @@ static inline int
 current_thread_holds_gil(struct _gil_runtime_state *gil, PyThreadState *tstate)
 {
     if (((PyThreadState*)_Py_atomic_load_ptr_relaxed(&gil->last_holder)) != tstate) {
+#ifdef Py_GIL_DISABLED
+        assert(!tstate->_status.holds_gil);
+#endif
         return 0;
     }
-    return _Py_atomic_load_int_relaxed(&gil->locked);
+    int locked = _Py_atomic_load_int_relaxed(&gil->locked);
+#ifdef Py_GIL_DISABLED
+    assert(!tstate->_status.holds_gil || locked);
+#endif
+    return locked;
 }
 #endif
 
@@ -563,23 +572,24 @@ PyEval_ReleaseLock(void)
     /* This function must succeed when the current thread state is NULL.
        We therefore avoid PyThreadState_Get() which dumps a fatal error
        in debug mode. */
-    drop_gil(tstate->interp, tstate);
+    drop_gil(tstate->interp, tstate, 0);
 }
 
-int
+void
 _PyEval_AcquireLock(PyThreadState *tstate)
 {
     _Py_EnsureTstateNotNULL(tstate);
-    return take_gil(tstate);
+    take_gil(tstate);
 }
 
 void
-_PyEval_ReleaseLock(PyInterpreterState *interp, PyThreadState *tstate)
+_PyEval_ReleaseLock(PyInterpreterState *interp,
+                    PyThreadState *tstate,
+                    int thread_dying)
 {
-    /* If tstate is NULL then we do not expect the current thread
-       to acquire the GIL ever again. */
-    assert(tstate == NULL || tstate->interp == interp);
-    drop_gil(interp, tstate);
+    assert(tstate != NULL);
+    assert(tstate->interp == interp);
+    drop_gil(interp, tstate, thread_dying);
 }
 
 void
@@ -1136,7 +1146,7 @@ _PyEval_DisableGIL(PyThreadState *tstate)
         //
         // Drop the GIL, which will wake up any threads waiting in take_gil()
         // and let them resume execution without the GIL.
-        drop_gil_impl(gil);
+        drop_gil_impl(tstate, gil);
         return 1;
     }
     return 0;

@@ -1831,21 +1831,10 @@ _PyThreadState_DeleteCurrent(PyThreadState *tstate)
     _Py_EnsureTstateNotNULL(tstate);
 #ifdef Py_GIL_DISABLED
     _Py_qsbr_detach(((_PyThreadStateImpl *)tstate)->qsbr);
-    // tstate_delete_common() removes tstate from consideration for
-    // stop-the-worlds. This means that another thread could enable the GIL
-    // before our call to _PyEval_ReleaseLock(), violating its invariant that
-    // the calling thread holds the GIL if and only if the GIL is enabled. Deal
-    // with this by deciding if we need to release the GIL before
-    // tstate_delete_common(), when the invariant is still true.
-    int holds_gil = _PyEval_IsGILEnabled(tstate);
-#else
-    int holds_gil = 1;
 #endif
     current_fast_clear(tstate->interp->runtime);
     tstate_delete_common(tstate);
-    if (holds_gil) {
-        _PyEval_ReleaseLock(tstate->interp, NULL);
-    }
+    _PyEval_ReleaseLock(tstate->interp, tstate, 1);
     free_threadstate((_PyThreadStateImpl *)tstate);
 }
 
@@ -2070,7 +2059,7 @@ _PyThreadState_Attach(PyThreadState *tstate)
 
 
     while (1) {
-        int acquired_gil = _PyEval_AcquireLock(tstate);
+        _PyEval_AcquireLock(tstate);
 
         // XXX assert(tstate_is_alive(tstate));
         current_fast_set(&_PyRuntime, tstate);
@@ -2081,20 +2070,18 @@ _PyThreadState_Attach(PyThreadState *tstate)
         }
 
 #ifdef Py_GIL_DISABLED
-        if (_PyEval_IsGILEnabled(tstate) != acquired_gil) {
+        if (_PyEval_IsGILEnabled(tstate) != tstate->_status.holds_gil) {
             // The GIL was enabled between our call to _PyEval_AcquireLock()
             // and when we attached (the GIL can't go from enabled to disabled
             // here because only a thread holding the GIL can disable
             // it). Detach and try again.
-            assert(!acquired_gil);
+            assert(!tstate->_status.holds_gil);
             tstate_set_detached(tstate, _Py_THREAD_DETACHED);
             tstate_deactivate(tstate);
             current_fast_clear(&_PyRuntime);
             continue;
         }
         _Py_qsbr_attach(((_PyThreadStateImpl *)tstate)->qsbr);
-#else
-        (void)acquired_gil;
 #endif
         break;
     }
@@ -2125,7 +2112,7 @@ detach_thread(PyThreadState *tstate, int detached_state)
     tstate_deactivate(tstate);
     tstate_set_detached(tstate, detached_state);
     current_fast_clear(&_PyRuntime);
-    _PyEval_ReleaseLock(tstate->interp, tstate);
+    _PyEval_ReleaseLock(tstate->interp, tstate, 0);
 }
 
 void