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Commit441affc

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gh-111964: Implement stop-the-world pauses (gh-112471)
The `--disable-gil` builds occasionally need to pause all but one thread. Someexamples include:* Cyclic garbage collection, where this is often called a "stop the world event"* Before calling `fork()`, to ensure a consistent state for internal data structures* During interpreter shutdown, to ensure that daemon threads aren't accessing Python objectsThis adds the following functions to implement global and per-interpreter pauses:* `_PyEval_StopTheWorldAll()` and `_PyEval_StartTheWorldAll()` (for the global runtime)* `_PyEval_StopTheWorld()` and `_PyEval_StartTheWorld()` (per-interpreter)(The function names may change.)These functions are no-ops outside of the `--disable-gil` build.
1 parent5f19978 commit441affc

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10 files changed

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-29
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10 files changed

+336
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lines changed

‎Include/cpython/pystate.h‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -102,7 +102,7 @@ struct _ts {
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#endif
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int_whence;
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105-
/* Thread state (_Py_THREAD_ATTACHED, _Py_THREAD_DETACHED,_Py_THREAD_GC).
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/* Thread state (_Py_THREAD_ATTACHED, _Py_THREAD_DETACHED,_Py_THREAD_SUSPENDED).
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See Include/internal/pycore_pystate.h for more details. */
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intstate;
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‎Include/internal/pycore_ceval.h‎

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Original file line numberDiff line numberDiff line change
@@ -205,6 +205,7 @@ void _PyEval_FrameClearAndPop(PyThreadState *tstate, _PyInterpreterFrame *frame)
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#define_PY_CALLS_TO_DO_BIT 2
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#define_PY_ASYNC_EXCEPTION_BIT 3
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#define_PY_GC_SCHEDULED_BIT 4
208+
#define_PY_EVAL_PLEASE_STOP_BIT 5
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/* Reserve a few bits for future use */
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#define_PY_EVAL_EVENTS_BITS 8

‎Include/internal/pycore_interp.h‎

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@@ -41,6 +41,22 @@ struct _Py_long_state {
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intmax_str_digits;
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};
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// Support for stop-the-world events. This exists in both the PyRuntime struct
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// for global pauses and in each PyInterpreterState for per-interpreter pauses.
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struct_stoptheworld_state {
47+
PyMutexmutex;// Serializes stop-the-world attempts.
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// NOTE: The below fields are protected by HEAD_LOCK(runtime), not by the
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// above mutex.
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boolrequested;// Set when a pause is requested.
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boolworld_stopped;// Set when the world is stopped.
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boolis_global;// Set when contained in PyRuntime struct.
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PyEventstop_event;// Set when thread_countdown reaches zero.
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Py_ssize_tthread_countdown;// Number of threads that must pause.
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PyThreadState*requester;// Thread that requested the pause (may be NULL).
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};
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/* cross-interpreter data registry */
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@@ -166,6 +182,7 @@ struct _is {
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struct_warnings_runtime_statewarnings;
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structatexit_stateatexit;
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struct_stoptheworld_statestoptheworld;
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#if defined(Py_GIL_DISABLED)
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struct_mimalloc_interp_statemimalloc;

‎Include/internal/pycore_llist.h‎

Lines changed: 1 addition & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -37,8 +37,7 @@ struct llist_node {
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};
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// Get the struct containing a node.
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#definellist_data(node,type,member) \
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(type*)((char*)node - offsetof(type, member))
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#definellist_data(node,type,member) (_Py_CONTAINER_OF(node, type, member))
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// Iterate over a list.
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#definellist_for_each(node,head) \

‎Include/internal/pycore_pystate.h‎

Lines changed: 39 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -21,23 +21,27 @@ extern "C" {
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// interpreter at the same time. Only the "bound" thread may perform the
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// transitions between "attached" and "detached" on its own PyThreadState.
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//
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// The "gc" state is used to implement stop-the-world pauses, such as for
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// cyclic garbage collection. It is only used in `--disable-gil` builds. It is
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// similar to the "detached" state, but only the thread performing a
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//stop-the-world pause may transition threads between the "detached" and "gc"
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//states. A thread trying to "attach" from the "gc" state will block until
29-
//it is transitioned back to "detached" when thestop-the-world pauseis
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//complete.
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// The "suspended" state is used to implement stop-the-world pauses, such as
25+
//forcyclic garbage collection. It is only used in `--disable-gil` builds.
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//The "suspended" state issimilar to the "detached" state in that in both
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//statesthe thread is not allowed to call most Python APIs. However, unlike
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//the "detached" state, a thread may not transition itself out from the
29+
//"suspended" state. Only the thread performing astop-the-world pausemay
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//transition a thread from the "suspended" state back to the "detached" state.
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//
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// State transition diagram:
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//
3434
// (bound thread) (stop-the-world thread)
35-
// [attached] <-> [detached] <-> [gc]
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// [attached] <-> [detached] <-> [suspended]
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// | ^
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// +---------------------------->---------------------------+
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// (bound thread)
3639
//
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// See `_PyThreadState_Attach()` and `_PyThreadState_Detach()`.
40+
// The (bound thread) and (stop-the-world thread) labels indicate which thread
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// is allowed to perform the transition.
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#define_Py_THREAD_DETACHED 0
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#define_Py_THREAD_ATTACHED 1
40-
#define_Py_THREAD_GC 2
44+
#define_Py_THREAD_SUSPENDED 2
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/* Check if the current thread is the main thread.
@@ -140,13 +144,36 @@ _PyThreadState_GET(void)
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//
141145
// High-level code should generally call PyEval_RestoreThread() instead, which
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// calls this function.
143-
void_PyThreadState_Attach(PyThreadState*tstate);
147+
externvoid_PyThreadState_Attach(PyThreadState*tstate);
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// Detaches the current thread from the interpreter.
146150
//
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// High-level code should generally call PyEval_SaveThread() instead, which
148152
// calls this function.
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void_PyThreadState_Detach(PyThreadState*tstate);
153+
externvoid_PyThreadState_Detach(PyThreadState*tstate);
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// Detaches the current thread to the "suspended" state if a stop-the-world
156+
// pause is in progress.
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//
158+
// If there is no stop-the-world pause in progress, then the thread switches
159+
// to the "detached" state.
160+
externvoid_PyThreadState_Suspend(PyThreadState*tstate);
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162+
// Perform a stop-the-world pause for all threads in the all interpreters.
163+
//
164+
// Threads in the "attached" state are paused and transitioned to the "GC"
165+
// state. Threads in the "detached" state switch to the "GC" state, preventing
166+
// them from reattaching until the stop-the-world pause is complete.
167+
//
168+
// NOTE: This is a no-op outside of Py_GIL_DISABLED builds.
169+
externvoid_PyEval_StopTheWorldAll(_PyRuntimeState*runtime);
170+
externvoid_PyEval_StartTheWorldAll(_PyRuntimeState*runtime);
171+
172+
// Perform a stop-the-world pause for threads in the specified interpreter.
173+
//
174+
// NOTE: This is a no-op outside of Py_GIL_DISABLED builds.
175+
externvoid_PyEval_StopTheWorld(PyInterpreterState*interp);
176+
externvoid_PyEval_StartTheWorld(PyInterpreterState*interp);
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staticinlinevoid

‎Include/internal/pycore_runtime.h‎

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Original file line numberDiff line numberDiff line change
@@ -227,6 +227,13 @@ typedef struct pyruntimestate {
227227
struct_faulthandler_runtime_statefaulthandler;
228228
struct_tracemalloc_runtime_statetracemalloc;
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230+
// The rwmutex is used to prevent overlapping global and per-interpreter
231+
// stop-the-world events. Global stop-the-world events lock the mutex
232+
// exclusively (as a "writer"), while per-interpreter stop-the-world events
233+
// lock it non-exclusively (as "readers").
234+
_PyRWMutexstoptheworld_mutex;
235+
struct_stoptheworld_statestoptheworld;
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230237
PyPreConfigpreconfig;
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// Audit values must be preserved when Py_Initialize()/Py_Finalize()

‎Include/internal/pycore_runtime_init.h‎

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Original file line numberDiff line numberDiff line change
@@ -116,6 +116,9 @@ extern PyTypeObject _PyExc_MemoryError;
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}, \
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.faulthandler = _faulthandler_runtime_state_INIT, \
118118
.tracemalloc = _tracemalloc_runtime_state_INIT, \
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.stoptheworld = { \
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.is_global = 1, \
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}, \
119122
.float_state = { \
120123
.float_format = _py_float_format_unknown, \
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.double_format = _py_float_format_unknown, \

‎Include/pymacro.h‎

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@@ -160,6 +160,9 @@
160160
Py_FatalError("Unreachable C code path reached")
161161
#endif
162162

163+
#define_Py_CONTAINER_OF(ptr,type,member) \
164+
(type*)((char*)ptr - offsetof(type, member))
165+
163166
// Prevent using an expression as a l-value.
164167
// For example, "int x; _Py_RVALUE(x) = 1;" fails with a compiler error.
165168
#define_Py_RVALUE(EXPR) ((void)0, (EXPR))

‎Python/ceval_gil.c‎

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Original file line numberDiff line numberDiff line change
@@ -949,6 +949,15 @@ _Py_HandlePending(PyThreadState *tstate)
949949
{
950950
PyInterpreterState*interp=tstate->interp;
951951

952+
/* Stop-the-world */
953+
if (_Py_eval_breaker_bit_is_set(interp,_PY_EVAL_PLEASE_STOP_BIT)) {
954+
_Py_set_eval_breaker_bit(interp,_PY_EVAL_PLEASE_STOP_BIT,0);
955+
_PyThreadState_Suspend(tstate);
956+
957+
/* The attach blocks until the stop-the-world event is complete. */
958+
_PyThreadState_Attach(tstate);
959+
}
960+
952961
/* Pending signals */
953962
if (_Py_eval_breaker_bit_is_set(interp,_PY_SIGNALS_PENDING_BIT)) {
954963
if (handle_signals(tstate)!=0) {

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