cpython/Python/ceval_gil.h at 3.11 · python/cpython · GitHub

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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);

}

#ifdef HAVE_FORK

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);

}

#endif

static void

drop_gil(struct _ceval_runtime_state *ceval, struct _ceval_state *ceval2,

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(&ceval2->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(_PyThreadState_CheckConsistency(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

}

/* 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 (_PyThreadState_MustExit(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(_PyThreadState_CheckConsistency(tstate));

PyInterpreterState *interp = tstate->interp;

struct _ceval_runtime_state *ceval = &interp->runtime->ceval;

struct _ceval_state *ceval2 = &interp->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;

}

int drop_requested = 0;

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 (_PyThreadState_MustExit(tstate)) {

MUTEX_UNLOCK(gil->mutex);

// gh-96387: If the loop requested a drop request in a previous

// iteration, reset the request. Otherwise, drop_gil() can

// block forever waiting for the thread which exited. Drop

// requests made by other threads are also reset: these threads

// may have to request again a drop request (iterate one more

// time).

if (drop_requested) {

RESET_GIL_DROP_REQUEST(interp);

}

PyThread_exit_thread();

}

assert(_PyThreadState_CheckConsistency(tstate));

SET_GIL_DROP_REQUEST(interp);

drop_requested = 1;

}

_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 (_PyThreadState_MustExit(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, ceval2, tstate);

PyThread_exit_thread();

}

assert(_PyThreadState_CheckConsistency(tstate));

if (_Py_atomic_load_relaxed(&ceval2->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(). */

COMPUTE_EVAL_BREAKER(interp, ceval, ceval2);

}

/* Don't access tstate if the thread must exit */

if (tstate->async_exc != NULL) {

_PyEval_SignalAsyncExc(tstate->interp);

}

MUTEX_UNLOCK(gil->mutex);

errno = err;

}

void _PyEval_SetSwitchInterval(unsigned long microseconds)

{

struct _gil_runtime_state *gil = &_PyRuntime.ceval.gil;

gil->interval = microseconds;

}

unsigned long _PyEval_GetSwitchInterval()

{

struct _gil_runtime_state *gil = &_PyRuntime.ceval.gil;

return gil->interval;

}

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