mirror of
https://github.com/python/cpython.git
synced 2025-10-24 18:33:49 +00:00
e75ff35af2
Extract cross-platform condition variable support into a separate file and provide user-mode non-recursive locks for Windows.
270 lines
8.9 KiB
C
270 lines
8.9 KiB
C
/*
|
|
* Implementation of the Global Interpreter Lock (GIL).
|
|
*/
|
|
|
|
#include <stdlib.h>
|
|
#include <errno.h>
|
|
|
|
|
|
/* First some general settings */
|
|
|
|
/* microseconds (the Python API uses seconds, though) */
|
|
#define DEFAULT_INTERVAL 5000
|
|
static unsigned long gil_interval = DEFAULT_INTERVAL;
|
|
#define INTERVAL (gil_interval >= 1 ? gil_interval : 1)
|
|
|
|
/* Enable if you want to force the switching of threads at least every `gil_interval` */
|
|
#undef FORCE_SWITCHING
|
|
#define FORCE_SWITCHING
|
|
|
|
|
|
/*
|
|
Notes about the implementation:
|
|
|
|
- The GIL is just a boolean variable (gil_locked) whose access is protected
|
|
by a mutex (gil_mutex), and whose changes are signalled by a condition
|
|
variable (gil_cond). gil_mutex is taken for short periods of time,
|
|
and therefore mostly uncontended.
|
|
|
|
- In the GIL-holding thread, the main loop (PyEval_EvalFrameEx) must be
|
|
able to release the GIL on demand by another thread. A volatile boolean
|
|
variable (gil_drop_request) is used for that purpose, which is checked
|
|
at every turn of the eval loop. That variable is set after a wait of
|
|
`interval` microseconds on `gil_cond` has timed out.
|
|
|
|
[Actually, another volatile boolean variable (eval_breaker) is used
|
|
which ORs several conditions into one. Volatile booleans are
|
|
sufficient as inter-thread signalling means since Python is run
|
|
on cache-coherent architectures only.]
|
|
|
|
- A thread wanting to take the GIL will first let pass a given amount of
|
|
time (`interval` microseconds) before setting gil_drop_request. This
|
|
encourages a defined switching period, but doesn't enforce it since
|
|
opcodes can take an arbitrary time to execute.
|
|
|
|
The `interval` value is available for the user to read and modify
|
|
using the Python API `sys.{get,set}switchinterval()`.
|
|
|
|
- When a thread releases the GIL and gil_drop_request is set, that thread
|
|
ensures that another GIL-awaiting thread gets scheduled.
|
|
It does so by waiting on a condition variable (switch_cond) until
|
|
the value of gil_last_holder is changed to something else than its
|
|
own thread state pointer, indicating that another thread was able to
|
|
take the GIL.
|
|
|
|
This is meant to prohibit the latency-adverse behaviour on multi-core
|
|
machines where one thread would speculatively release the GIL, but still
|
|
run and end up being the first to re-acquire it, making the "timeslices"
|
|
much longer than expected.
|
|
(Note: this mechanism is enabled with FORCE_SWITCHING above)
|
|
*/
|
|
|
|
#include "condvar.h"
|
|
#ifndef Py_HAVE_CONDVAR
|
|
#error You need either a POSIX-compatible or a Windows system!
|
|
#endif
|
|
|
|
#define MUTEX_T PyMUTEX_T
|
|
#define MUTEX_INIT(mut) \
|
|
if (PyMUTEX_INIT(&(mut))) { \
|
|
Py_FatalError("PyMUTEX_INIT(" #mut ") failed"); };
|
|
#define MUTEX_FINI(mut) \
|
|
if (PyMUTEX_FINI(&(mut))) { \
|
|
Py_FatalError("PyMUTEX_FINI(" #mut ") failed"); };
|
|
#define MUTEX_LOCK(mut) \
|
|
if (PyMUTEX_LOCK(&(mut))) { \
|
|
Py_FatalError("PyMUTEX_LOCK(" #mut ") failed"); };
|
|
#define MUTEX_UNLOCK(mut) \
|
|
if (PyMUTEX_UNLOCK(&(mut))) { \
|
|
Py_FatalError("PyMUTEX_UNLOCK(" #mut ") failed"); };
|
|
|
|
#define COND_T PyCOND_T
|
|
#define COND_INIT(cond) \
|
|
if (PyCOND_INIT(&(cond))) { \
|
|
Py_FatalError("PyCOND_INIT(" #cond ") failed"); };
|
|
#define COND_FINI(cond) \
|
|
if (PyCOND_FINI(&(cond))) { \
|
|
Py_FatalError("PyCOND_FINI(" #cond ") failed"); };
|
|
#define COND_SIGNAL(cond) \
|
|
if (PyCOND_SIGNAL(&(cond))) { \
|
|
Py_FatalError("PyCOND_SIGNAL(" #cond ") failed"); };
|
|
#define COND_WAIT(cond, mut) \
|
|
if (PyCOND_WAIT(&(cond), &(mut))) { \
|
|
Py_FatalError("PyCOND_WAIT(" #cond ") failed"); };
|
|
#define COND_TIMED_WAIT(cond, mut, microseconds, timeout_result) \
|
|
{ \
|
|
int r = PyCOND_TIMEDWAIT(&(cond), &(mut), (microseconds)); \
|
|
if (r < 0) \
|
|
Py_FatalError("PyCOND_WAIT(" #cond ") failed"); \
|
|
if (r) /* 1 == timeout, 2 == impl. can't say, so assume timeout */ \
|
|
timeout_result = 1; \
|
|
else \
|
|
timeout_result = 0; \
|
|
} \
|
|
|
|
|
|
|
|
/* Whether the GIL is already taken (-1 if uninitialized). This is atomic
|
|
because it can be read without any lock taken in ceval.c. */
|
|
static _Py_atomic_int gil_locked = {-1};
|
|
/* Number of GIL switches since the beginning. */
|
|
static unsigned long gil_switch_number = 0;
|
|
/* Last PyThreadState holding / having held the GIL. This helps us know
|
|
whether anyone else was scheduled after we dropped the GIL. */
|
|
static _Py_atomic_address gil_last_holder = {NULL};
|
|
|
|
/* This condition variable allows one or several threads to wait until
|
|
the GIL is released. In addition, the mutex also protects the above
|
|
variables. */
|
|
static COND_T gil_cond;
|
|
static MUTEX_T gil_mutex;
|
|
|
|
#ifdef FORCE_SWITCHING
|
|
/* This condition variable helps the GIL-releasing thread wait for
|
|
a GIL-awaiting thread to be scheduled and take the GIL. */
|
|
static COND_T switch_cond;
|
|
static MUTEX_T switch_mutex;
|
|
#endif
|
|
|
|
|
|
static int gil_created(void)
|
|
{
|
|
return _Py_atomic_load_explicit(&gil_locked, _Py_memory_order_acquire) >= 0;
|
|
}
|
|
|
|
static void create_gil(void)
|
|
{
|
|
MUTEX_INIT(gil_mutex);
|
|
#ifdef FORCE_SWITCHING
|
|
MUTEX_INIT(switch_mutex);
|
|
#endif
|
|
COND_INIT(gil_cond);
|
|
#ifdef FORCE_SWITCHING
|
|
COND_INIT(switch_cond);
|
|
#endif
|
|
_Py_atomic_store_relaxed(&gil_last_holder, NULL);
|
|
_Py_ANNOTATE_RWLOCK_CREATE(&gil_locked);
|
|
_Py_atomic_store_explicit(&gil_locked, 0, _Py_memory_order_release);
|
|
}
|
|
|
|
static void destroy_gil(void)
|
|
{
|
|
/* some pthread-like implementations tie the mutex to the cond
|
|
* and must have the cond destroyed first.
|
|
*/
|
|
COND_FINI(gil_cond);
|
|
MUTEX_FINI(gil_mutex);
|
|
#ifdef FORCE_SWITCHING
|
|
COND_FINI(switch_cond);
|
|
MUTEX_FINI(switch_mutex);
|
|
#endif
|
|
_Py_atomic_store_explicit(&gil_locked, -1, _Py_memory_order_release);
|
|
_Py_ANNOTATE_RWLOCK_DESTROY(&gil_locked);
|
|
}
|
|
|
|
static void recreate_gil(void)
|
|
{
|
|
_Py_ANNOTATE_RWLOCK_DESTROY(&gil_locked);
|
|
/* XXX should we destroy the old OS resources here? */
|
|
create_gil();
|
|
}
|
|
|
|
static void drop_gil(PyThreadState *tstate)
|
|
{
|
|
if (!_Py_atomic_load_relaxed(&gil_locked))
|
|
Py_FatalError("drop_gil: GIL is not locked");
|
|
/* tstate is allowed to be NULL (early interpreter init) */
|
|
if (tstate != NULL) {
|
|
/* 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_relaxed(&gil_last_holder, tstate);
|
|
}
|
|
|
|
MUTEX_LOCK(gil_mutex);
|
|
_Py_ANNOTATE_RWLOCK_RELEASED(&gil_locked, /*is_write=*/1);
|
|
_Py_atomic_store_relaxed(&gil_locked, 0);
|
|
COND_SIGNAL(gil_cond);
|
|
MUTEX_UNLOCK(gil_mutex);
|
|
|
|
#ifdef FORCE_SWITCHING
|
|
if (_Py_atomic_load_relaxed(&gil_drop_request) && tstate != NULL) {
|
|
MUTEX_LOCK(switch_mutex);
|
|
/* Not switched yet => wait */
|
|
if (_Py_atomic_load_relaxed(&gil_last_holder) == tstate) {
|
|
RESET_GIL_DROP_REQUEST();
|
|
/* NOTE: if COND_WAIT does not atomically start waiting when
|
|
releasing the mutex, another thread can run through, take
|
|
the GIL and drop it again, and reset the condition
|
|
before we even had a chance to wait for it. */
|
|
COND_WAIT(switch_cond, switch_mutex);
|
|
}
|
|
MUTEX_UNLOCK(switch_mutex);
|
|
}
|
|
#endif
|
|
}
|
|
|
|
static void take_gil(PyThreadState *tstate)
|
|
{
|
|
int err;
|
|
if (tstate == NULL)
|
|
Py_FatalError("take_gil: NULL tstate");
|
|
|
|
err = errno;
|
|
MUTEX_LOCK(gil_mutex);
|
|
|
|
if (!_Py_atomic_load_relaxed(&gil_locked))
|
|
goto _ready;
|
|
|
|
while (_Py_atomic_load_relaxed(&gil_locked)) {
|
|
int timed_out = 0;
|
|
unsigned long saved_switchnum;
|
|
|
|
saved_switchnum = gil_switch_number;
|
|
COND_TIMED_WAIT(gil_cond, gil_mutex, INTERVAL, timed_out);
|
|
/* If we timed out and no switch occurred in the meantime, it is time
|
|
to ask the GIL-holding thread to drop it. */
|
|
if (timed_out &&
|
|
_Py_atomic_load_relaxed(&gil_locked) &&
|
|
gil_switch_number == saved_switchnum) {
|
|
SET_GIL_DROP_REQUEST();
|
|
}
|
|
}
|
|
_ready:
|
|
#ifdef FORCE_SWITCHING
|
|
/* This mutex must be taken before modifying gil_last_holder (see drop_gil()). */
|
|
MUTEX_LOCK(switch_mutex);
|
|
#endif
|
|
/* We now hold the GIL */
|
|
_Py_atomic_store_relaxed(&gil_locked, 1);
|
|
_Py_ANNOTATE_RWLOCK_ACQUIRED(&gil_locked, /*is_write=*/1);
|
|
|
|
if (tstate != _Py_atomic_load_relaxed(&gil_last_holder)) {
|
|
_Py_atomic_store_relaxed(&gil_last_holder, tstate);
|
|
++gil_switch_number;
|
|
}
|
|
|
|
#ifdef FORCE_SWITCHING
|
|
COND_SIGNAL(switch_cond);
|
|
MUTEX_UNLOCK(switch_mutex);
|
|
#endif
|
|
if (_Py_atomic_load_relaxed(&gil_drop_request)) {
|
|
RESET_GIL_DROP_REQUEST();
|
|
}
|
|
if (tstate->async_exc != NULL) {
|
|
_PyEval_SignalAsyncExc();
|
|
}
|
|
|
|
MUTEX_UNLOCK(gil_mutex);
|
|
errno = err;
|
|
}
|
|
|
|
void _PyEval_SetSwitchInterval(unsigned long microseconds)
|
|
{
|
|
gil_interval = microseconds;
|
|
}
|
|
|
|
unsigned long _PyEval_GetSwitchInterval()
|
|
{
|
|
return gil_interval;
|
|
}
|