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cpython/Python/ceval_gil.h

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/*
* Implementation of the Global Interpreter Lock (GIL).
*/
#include <stdlib.h>
#include <errno.h>
#include "pycore_atomic.h"
/*
Notes about the implementation:
- The GIL is just a boolean variable (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 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"
#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_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; \
} \
#define DEFAULT_INTERVAL 5000
static void _gil_initialize(struct _gil_runtime_state *gil)
{
_Py_atomic_int uninitialized = {-1};
gil->locked = uninitialized;
gil->interval = DEFAULT_INTERVAL;
}
static int gil_created(struct _gil_runtime_state *gil)
{
return (_Py_atomic_load_explicit(&gil->locked, _Py_memory_order_acquire) >= 0);
}
static void create_gil(struct _gil_runtime_state *gil)
{
MUTEX_INIT(gil->mutex);
#ifdef FORCE_SWITCHING
MUTEX_INIT(gil->switch_mutex);
#endif
COND_INIT(gil->cond);
#ifdef FORCE_SWITCHING
COND_INIT(gil->switch_cond);
#endif
_Py_atomic_store_relaxed(&gil->last_holder, 0);
_Py_ANNOTATE_RWLOCK_CREATE(&gil->locked);
_Py_atomic_store_explicit(&gil->locked, 0, _Py_memory_order_release);
}
static void destroy_gil(struct _gil_runtime_state *gil)
{
/* 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(gil->switch_cond);
MUTEX_FINI(gil->switch_mutex);
#endif
_Py_atomic_store_explicit(&gil->locked, -1,
_Py_memory_order_release);
_Py_ANNOTATE_RWLOCK_DESTROY(&gil->locked);
}
static void recreate_gil(struct _gil_runtime_state *gil)
{
_Py_ANNOTATE_RWLOCK_DESTROY(&gil->locked);
/* XXX should we destroy the old OS resources here? */
create_gil(gil);
}
static void
drop_gil(struct _ceval_runtime_state *ceval, PyThreadState *tstate)
{
struct _gil_runtime_state *gil = &ceval->gil;
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, (uintptr_t)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(&ceval->gil_drop_request) && tstate != NULL) {
MUTEX_LOCK(gil->switch_mutex);
/* Not switched yet => wait */
if (((PyThreadState*)_Py_atomic_load_relaxed(&gil->last_holder)) == tstate)
{
assert(is_tstate_valid(tstate));
RESET_GIL_DROP_REQUEST(tstate->interp);
/* 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(gil->switch_cond, gil->switch_mutex);
}
MUTEX_UNLOCK(gil->switch_mutex);
}
#endif
}
/* Check if a Python thread must exit immediately, rather than taking the GIL
if Py_Finalize() has been called.
When this function is called by a daemon thread after Py_Finalize() has been
called, the GIL does no longer exist.
tstate must be non-NULL. */
static inline int
tstate_must_exit(PyThreadState *tstate)
{
/* bpo-39877: Access _PyRuntime directly rather than using
tstate->interp->runtime to support calls from Python daemon threads.
After Py_Finalize() has been called, tstate can be a dangling pointer:
point to PyThreadState freed memory. */
PyThreadState *finalizing = _PyRuntimeState_GetFinalizing(&_PyRuntime);
return (finalizing != NULL && finalizing != tstate);
}
/* Take the GIL.
The function saves errno at entry and restores its value at exit.
tstate must be non-NULL. */
static void
take_gil(PyThreadState *tstate)
{
int err = errno;
assert(tstate != NULL);
if (tstate_must_exit(tstate)) {
/* bpo-39877: If Py_Finalize() has been called and tstate is not the
thread which called Py_Finalize(), exit immediately the thread.
This code path can be reached by a daemon thread after Py_Finalize()
completes. In this case, tstate is a dangling pointer: points to
PyThreadState freed memory. */
PyThread_exit_thread();
}
assert(is_tstate_valid(tstate));
PyInterpreterState *interp = tstate->interp;
struct _ceval_runtime_state *ceval = &interp->runtime->ceval;
struct _gil_runtime_state *gil = &ceval->gil;
/* Check that _PyEval_InitThreads() was called to create the lock */
assert(gil_created(gil));
MUTEX_LOCK(gil->mutex);
if (!_Py_atomic_load_relaxed(&gil->locked)) {
goto _ready;
}
while (_Py_atomic_load_relaxed(&gil->locked)) {
unsigned long saved_switchnum = gil->switch_number;
unsigned long interval = (gil->interval >= 1 ? gil->interval : 1);
int timed_out = 0;
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)
{
if (tstate_must_exit(tstate)) {
MUTEX_UNLOCK(gil->mutex);
PyThread_exit_thread();
}
assert(is_tstate_valid(tstate));
SET_GIL_DROP_REQUEST(interp);
}
}
_ready:
#ifdef FORCE_SWITCHING
/* This mutex must be taken before modifying gil->last_holder:
see drop_gil(). */
MUTEX_LOCK(gil->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 != (PyThreadState*)_Py_atomic_load_relaxed(&gil->last_holder)) {
_Py_atomic_store_relaxed(&gil->last_holder, (uintptr_t)tstate);
++gil->switch_number;
}
#ifdef FORCE_SWITCHING
COND_SIGNAL(gil->switch_cond);
MUTEX_UNLOCK(gil->switch_mutex);
#endif
if (tstate_must_exit(tstate)) {
/* bpo-36475: If Py_Finalize() has been called and tstate is not
the thread which called Py_Finalize(), exit immediately the
thread.
This code path can be reached by a daemon thread which was waiting
in take_gil() while the main thread called
wait_for_thread_shutdown() from Py_Finalize(). */
MUTEX_UNLOCK(gil->mutex);
drop_gil(ceval, tstate);
PyThread_exit_thread();
}
assert(is_tstate_valid(tstate));
if (_Py_atomic_load_relaxed(&ceval->gil_drop_request)) {
RESET_GIL_DROP_REQUEST(interp);
}
else {
/* bpo-40010: eval_breaker should be recomputed to be set to 1 if there
is a pending signal: signal received by another thread which cannot
handle signals.
Note: RESET_GIL_DROP_REQUEST() calls COMPUTE_EVAL_BREAKER(). */
struct _ceval_state *ceval2 = &interp->ceval;
COMPUTE_EVAL_BREAKER(interp, ceval, ceval2);
}
/* Don't access tstate if the thread must exit */
if (tstate->async_exc != NULL) {
_PyEval_SignalAsyncExc(tstate);
}
MUTEX_UNLOCK(gil->mutex);
errno = err;
}
void _PyEval_SetSwitchInterval(unsigned long microseconds)
{
_PyRuntime.ceval.gil.interval = microseconds;
}
unsigned long _PyEval_GetSwitchInterval()
{
return _PyRuntime.ceval.gil.interval;
}
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