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Allow disabling the GIL with PYTHON_GIL=0
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In free-threaded builds, running with `PYTHON_GIL=0` will now disable the
GIL. # and # track follow-up work to re-enable the GIL when loading an
incompatible extension, and to disable the GIL by default.

In order to support re-enabling the GIL at runtime, all GIL-related data
structures are initialized as usual, and disabling the GIL simply sets a flag
that causes `take_gil()` and `drop_gil()` to return early.

With `PYTHON_GIL=0` set, I spot-checked a few tests and small programs that
don't use threads. They all seem to run fine, and very basic threaded programs
work, sometimes. Trying to run the full test suite crashes pretty quickly, in
`test_asyncio`.
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swtaarrs committed Mar 4, 2024
1 parent 88b5c66 commit ac24233
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Showing 5 changed files with 57 additions and 0 deletions.
3 changes: 3 additions & 0 deletions Include/cpython/initconfig.h
Original file line number Diff line number Diff line change
Expand Up @@ -181,6 +181,9 @@ typedef struct PyConfig {
int int_max_str_digits;

int cpu_count;
#ifdef Py_GIL_DISABLED
int enable_gil;
#endif

/* --- Path configuration inputs ------------ */
int pathconfig_warnings;
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5 changes: 5 additions & 0 deletions Include/internal/pycore_gil.h
Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,11 @@ extern "C" {
#define FORCE_SWITCHING

struct _gil_runtime_state {
#ifdef Py_GIL_DISABLED
/* Whether or not this GIL is being used. Can change from 0 to 1 at runtime
if, for example, a module that requires the GIL is loaded. */
int enabled;
#endif
/* microseconds (the Python API uses seconds, though) */
unsigned long interval;
/* Last PyThreadState holding / having held the GIL. This helps us
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12 changes: 12 additions & 0 deletions Include/internal/pycore_initconfig.h
Original file line number Diff line number Diff line change
Expand Up @@ -153,6 +153,18 @@ typedef enum {
_PyConfig_INIT_ISOLATED = 3
} _PyConfigInitEnum;

typedef enum {
/* For now, this means the GIL is enabled.
gh-116329: This will eventually change to "the GIL is disabled but can
be reenabled by loading an incompatible extension module." */
_PyConfig_GIL_DEFAULT,

/* The GIL has been forced off or on, and will not be affected by module loading. */
_PyConfig_GIL_DISABLE,
_PyConfig_GIL_ENABLE,
} _PyConfigGILEnum;

// Export for '_testembed' program
PyAPI_FUNC(void) _PyConfig_InitCompatConfig(PyConfig *config);

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15 changes: 15 additions & 0 deletions Python/ceval_gil.c
Original file line number Diff line number Diff line change
Expand Up @@ -219,6 +219,11 @@ drop_gil(PyInterpreterState *interp, PyThreadState *tstate)
// XXX assert(tstate == NULL || !tstate->_status.cleared);

struct _gil_runtime_state *gil = ceval->gil;
#ifdef Py_GIL_DISABLED
if (!gil->enabled) {
return;
}
#endif
if (!_Py_atomic_load_ptr_relaxed(&gil->locked)) {
Py_FatalError("drop_gil: GIL is not locked");
}
Expand Down Expand Up @@ -294,6 +299,11 @@ take_gil(PyThreadState *tstate)
assert(_PyThreadState_CheckConsistency(tstate));
PyInterpreterState *interp = tstate->interp;
struct _gil_runtime_state *gil = interp->ceval.gil;
#ifdef Py_GIL_DISABLED
if (!gil->enabled) {
return;
}
#endif

/* Check that _PyEval_InitThreads() was called to create the lock */
assert(gil_created(gil));
Expand Down Expand Up @@ -438,6 +448,11 @@ static void
init_own_gil(PyInterpreterState *interp, struct _gil_runtime_state *gil)
{
assert(!gil_created(gil));
#ifdef Py_GIL_DISABLED
// gh-116329: Once it is safe to do so, change this condition to
// (enable_gil == _PyConfig_GIL_ENABLE), so the GIL is disabled by default.
gil->enabled = _PyInterpreterState_GetConfig(interp)->enable_gil != _PyConfig_GIL_DISABLE;
#endif
create_gil(gil);
assert(gil_created(gil));
interp->ceval.gil = gil;
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22 changes: 22 additions & 0 deletions Python/initconfig.c
Original file line number Diff line number Diff line change
Expand Up @@ -93,6 +93,9 @@ static const PyConfigSpec PYCONFIG_SPEC[] = {
SPEC(safe_path, UINT),
SPEC(int_max_str_digits, INT),
SPEC(cpu_count, INT),
#ifdef Py_GIL_DISABLED
SPEC(enable_gil, INT),
#endif
SPEC(pathconfig_warnings, UINT),
SPEC(program_name, WSTR),
SPEC(pythonpath_env, WSTR_OPT),
Expand Down Expand Up @@ -276,6 +279,9 @@ static const char usage_envvars[] =
"PYTHON_CPU_COUNT: Overrides the return value of os.process_cpu_count(),\n"
" os.cpu_count(), and multiprocessing.cpu_count() if set to\n"
" a positive integer.\n"
#ifdef Py_GIL_DISABLED
"PYTHON_GIL : When set to 0, disables the GIL.\n"
#endif
"PYTHONDEVMODE : enable the development mode.\n"
"PYTHONPYCACHEPREFIX: root directory for bytecode cache (pyc) files.\n"
"PYTHONWARNDEFAULTENCODING: enable opt-in EncodingWarning for 'encoding=None'.\n"
Expand Down Expand Up @@ -860,6 +866,9 @@ _PyConfig_InitCompatConfig(PyConfig *config)
config->_is_python_build = 0;
config->code_debug_ranges = 1;
config->cpu_count = -1;
#ifdef Py_GIL_DISABLED
config->enable_gil = _PyConfig_GIL_DEFAULT;
#endif
}


Expand Down Expand Up @@ -1642,6 +1651,19 @@ config_read_env_vars(PyConfig *config)
config->safe_path = 1;
}

const char *gil = config_get_env(config, "PYTHON_GIL");
if (gil != NULL) {
#ifdef Py_GIL_DISABLED
if (strcmp(gil, "0") == 0) {
config->enable_gil = _PyConfig_GIL_DISABLE;
} else if (strcmp(gil, "1") != 0) {
return _PyStatus_ERR("PYTHON_GIL must be \"0\" or \"1\"");
}
#else
return _PyStatus_ERR("PYTHON_GIL is not supported by this build");
#endif
}

return _PyStatus_OK();
}

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