cpython/Python/lock.c at main · All-Blockchains/cpython · GitHub

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// Lock implementation

#include "Python.h"

#include "pycore_lock.h"

#include "pycore_parking_lot.h"

#include "pycore_semaphore.h"

#include "pycore_time.h" // _PyTime_Add()

#include "pycore_stats.h" // FT_STAT_MUTEX_SLEEP_INC()

#ifdef MS_WINDOWS

# ifndef WIN32_LEAN_AND_MEAN

# define WIN32_LEAN_AND_MEAN

# endif

# include <windows.h> // SwitchToThread()

#elif defined(HAVE_SCHED_H)

# include <sched.h> // sched_yield()

#endif

// If a thread waits on a lock for longer than TIME_TO_BE_FAIR_NS (1 ms), then

// the unlocking thread directly hands off ownership of the lock. This avoids

// starvation.

static const PyTime_t TIME_TO_BE_FAIR_NS = 1000*1000;

// Spin for a bit before parking the thread. This is only enabled for

// `--disable-gil` builds because it is unlikely to be helpful if the GIL is

// enabled.

#if Py_GIL_DISABLED

static const int MAX_SPIN_COUNT = 40;

static const int RELOAD_SPIN_MASK = 3;

#else

static const int MAX_SPIN_COUNT = 0;

static const int RELOAD_SPIN_MASK = 1;

#endif

struct mutex_entry {

// The time after which the unlocking thread should hand off lock ownership

// directly to the waiting thread. Written by the waiting thread.

PyTime_t time_to_be_fair;

// Set to 1 if the lock was handed off. Written by the unlocking thread.

int handed_off;

};

void

_Py_yield(void)

{

#ifdef MS_WINDOWS

SwitchToThread();

#elif defined(HAVE_SCHED_H)

sched_yield();

#endif

}

PyLockStatus

_PyMutex_LockTimed(PyMutex *m, PyTime_t timeout, _PyLockFlags flags)

{

uint8_t v = _Py_atomic_load_uint8_relaxed(&m->_bits);

if ((v & _Py_LOCKED) == 0) {

if (_Py_atomic_compare_exchange_uint8(&m->_bits, &v, v|_Py_LOCKED)) {

return PY_LOCK_ACQUIRED;

}

if (timeout == 0) {

return PY_LOCK_FAILURE;

}

FT_STAT_MUTEX_SLEEP_INC();

PyTime_t now;

// silently ignore error: cannot report error to the caller

(void)PyTime_MonotonicRaw(&now);

PyTime_t endtime = 0;

if (timeout > 0) {

endtime = _PyTime_Add(now, timeout);

}

struct mutex_entry entry = {

.time_to_be_fair = now + TIME_TO_BE_FAIR_NS,

.handed_off = 0,

};

Py_ssize_t spin_count = 0;

#ifdef Py_GIL_DISABLED

// Using thread-id as a way of reducing contention further in the reload below.

// It adds a pseudo-random starting offset to the recurrence, so that threads

// are less likely to try and run compare-exchange at the same time.

// The lower bits of platform thread ids are likely to not be random,

// hence the right shift.

const Py_ssize_t tid = (Py_ssize_t)(_Py_ThreadId() >> 12);

#else

const Py_ssize_t tid = 0;

#endif

for (;;) {

if ((v & _Py_LOCKED) == 0) {

// The lock is unlocked. Try to grab it.

if (_Py_atomic_compare_exchange_uint8(&m->_bits, &v, v|_Py_LOCKED)) {

return PY_LOCK_ACQUIRED;

}

continue;

}

if (!(v & _Py_HAS_PARKED) && spin_count < MAX_SPIN_COUNT) {

// Spin for a bit.

_Py_yield();

spin_count++;

if (((spin_count + tid) & RELOAD_SPIN_MASK) == 0) {

v = _Py_atomic_load_uint8_relaxed(&m->_bits);

}

continue;

}

if (timeout == 0) {

return PY_LOCK_FAILURE;

}

if ((flags & _PY_LOCK_PYTHONLOCK) && Py_IsFinalizing()) {

// At this phase of runtime shutdown, only the finalization thread

// can have attached thread state; others hang if they try

// attaching. And since operations on this lock requires attached

// thread state (_PY_LOCK_PYTHONLOCK), the finalization thread is

// running this code, and no other thread can unlock.

// Raise rather than hang. (_PY_LOCK_PYTHONLOCK allows raising

// exceptons.)

PyErr_SetString(PyExc_PythonFinalizationError,

"cannot acquire lock at interpreter finalization");

return PY_LOCK_FAILURE;

}

uint8_t newv = v;

if (!(v & _Py_HAS_PARKED)) {

// We are the first waiter. Set the _Py_HAS_PARKED flag.

newv = v | _Py_HAS_PARKED;

if (!_Py_atomic_compare_exchange_uint8(&m->_bits, &v, newv)) {

continue;

}

int ret = _PyParkingLot_Park(&m->_bits, &newv, sizeof(newv), timeout,

&entry, (flags & _PY_LOCK_DETACH) != 0);

if (ret == Py_PARK_OK) {

if (entry.handed_off) {

// We own the lock now. thread.Lock allows other threads

// to concurrently release the lock so we cannot assert that

// it is locked if _PY_LOCK_PYTHONLOCK is set.

assert(_Py_atomic_load_uint8_relaxed(&m->_bits) & _Py_LOCKED ||

(flags & _PY_LOCK_PYTHONLOCK) != 0);

return PY_LOCK_ACQUIRED;

}

else if (ret == Py_PARK_INTR && (flags & _PY_LOCK_HANDLE_SIGNALS)) {

if (Py_MakePendingCalls() < 0) {

return PY_LOCK_INTR;

}

else if (ret == Py_PARK_INTR && (flags & _PY_FAIL_IF_INTERRUPTED)) {

return PY_LOCK_INTR;

}

else if (ret == Py_PARK_TIMEOUT) {

assert(timeout >= 0);

return PY_LOCK_FAILURE;

}

if (timeout > 0) {

timeout = _PyDeadline_Get(endtime);

if (timeout <= 0) {

// Avoid negative values because those mean block forever.

timeout = 0;

}

v = _Py_atomic_load_uint8_relaxed(&m->_bits);

}

static void

mutex_unpark(void *arg, void *park_arg, int has_more_waiters)

{

PyMutex *m = (PyMutex*)arg;

struct mutex_entry *entry = (struct mutex_entry*)park_arg;

uint8_t v = 0;

if (entry) {

PyTime_t now;

// silently ignore error: cannot report error to the caller

(void)PyTime_MonotonicRaw(&now);

int should_be_fair = now > entry->time_to_be_fair;

entry->handed_off = should_be_fair;

if (should_be_fair) {

v |= _Py_LOCKED;

}

if (has_more_waiters) {

v |= _Py_HAS_PARKED;

}

_Py_atomic_store_uint8(&m->_bits, v);

}

int

_PyMutex_TryUnlock(PyMutex *m)

{

uint8_t v = _Py_atomic_load_uint8(&m->_bits);

for (;;) {

if ((v & _Py_LOCKED) == 0) {

// error: the mutex is not locked

return -1;

}

else if ((v & _Py_HAS_PARKED)) {

// wake up a single thread

_PyParkingLot_Unpark(&m->_bits, mutex_unpark, m);

return 0;

}

else if (_Py_atomic_compare_exchange_uint8(&m->_bits, &v, _Py_UNLOCKED)) {

// fast-path: no waiters

return 0;

}

// _PyRawMutex stores a linked list of `struct raw_mutex_entry`, one for each

// thread waiting on the mutex, directly in the mutex itself.

struct raw_mutex_entry {

struct raw_mutex_entry *next;

_PySemaphore sema;

};

void

_PyRawMutex_LockSlow(_PyRawMutex *m)

{

struct raw_mutex_entry waiter;

_PySemaphore_Init(&waiter.sema);

uintptr_t v = _Py_atomic_load_uintptr(&m->v);

for (;;) {

if ((v & _Py_LOCKED) == 0) {

// Unlocked: try to grab it (even if it has a waiter).

if (_Py_atomic_compare_exchange_uintptr(&m->v, &v, v|_Py_LOCKED)) {

break;

}

continue;

}

// Locked: try to add ourselves as a waiter.

waiter.next = (struct raw_mutex_entry *)(v & ~1);

uintptr_t desired = ((uintptr_t)&waiter)|_Py_LOCKED;

if (!_Py_atomic_compare_exchange_uintptr(&m->v, &v, desired)) {

continue;

}

// Wait for us to be woken up. Note that we still have to lock the

// mutex ourselves: it is NOT handed off to us.

// Loop until we observe an actual wakeup. A return of Py_PARK_INTR

// could otherwise let us exit _PySemaphore_Wait and destroy

// `waiter.sema` while _PyRawMutex_UnlockSlow's matching

// _PySemaphore_Wakeup is still pending, since the unlocker has

// already CAS-removed us from the waiter list without any handshake.

int res;

do {

res = _PySemaphore_Wait(&waiter.sema, -1);

} while (res != Py_PARK_OK);

}

_PySemaphore_Destroy(&waiter.sema);

}

void

_PyRawMutex_UnlockSlow(_PyRawMutex *m)

{

uintptr_t v = _Py_atomic_load_uintptr(&m->v);

for (;;) {

if ((v & _Py_LOCKED) == 0) {

Py_FatalError("unlocking mutex that is not locked");

}

struct raw_mutex_entry *waiter = (struct raw_mutex_entry *)(v & ~1);

if (waiter) {

uintptr_t next_waiter = (uintptr_t)waiter->next;

if (_Py_atomic_compare_exchange_uintptr(&m->v, &v, next_waiter)) {

_PySemaphore_Wakeup(&waiter->sema);

return;

}

else {

if (_Py_atomic_compare_exchange_uintptr(&m->v, &v, _Py_UNLOCKED)) {

return;

}

int

_PyEvent_IsSet(PyEvent *evt)

{

uint8_t v = _Py_atomic_load_uint8(&evt->v);

return v == _Py_LOCKED;

}

void

_PyEvent_Notify(PyEvent *evt)

{

uintptr_t v = _Py_atomic_exchange_uint8(&evt->v, _Py_LOCKED);

if (v == _Py_UNLOCKED) {

// no waiters

return;

}

else if (v == _Py_LOCKED) {

// event already set

return;

}

else {

assert(v == _Py_HAS_PARKED);

_PyParkingLot_UnparkAll(&evt->v);

}

void

PyEvent_Wait(PyEvent *evt)

{

while (!PyEvent_WaitTimed(evt, -1, /*detach=*/1))

;

}

int

PyEvent_WaitTimed(PyEvent *evt, PyTime_t timeout_ns, int detach)

{

for (;;) {

uint8_t v = _Py_atomic_load_uint8(&evt->v);

if (v == _Py_LOCKED) {

// event already set

return 1;

}

if (v == _Py_UNLOCKED) {

if (!_Py_atomic_compare_exchange_uint8(&evt->v, &v, _Py_HAS_PARKED)) {

continue;

}

uint8_t expected = _Py_HAS_PARKED;

(void) _PyParkingLot_Park(&evt->v, &expected, sizeof(evt->v),

timeout_ns, NULL, detach);

return _Py_atomic_load_uint8(&evt->v) == _Py_LOCKED;

}

static int

unlock_once(_PyOnceFlag *o, int res)

{

// On success (res=0), we set the state to _Py_ONCE_INITIALIZED.

// On failure (res=-1), we reset the state to _Py_UNLOCKED.

uint8_t new_value;

switch (res) {

case -1: new_value = _Py_UNLOCKED; break;

case 0: new_value = _Py_ONCE_INITIALIZED; break;

default: {

Py_FatalError("invalid result from _PyOnceFlag_CallOnce");

Py_UNREACHABLE();

break;

}

uint8_t old_value = _Py_atomic_exchange_uint8(&o->v, new_value);

if ((old_value & _Py_HAS_PARKED) != 0) {

// wake up anyone waiting on the once flag

_PyParkingLot_UnparkAll(&o->v);

}

return res;

}

int

_PyOnceFlag_CallOnceSlow(_PyOnceFlag *flag, _Py_once_fn_t *fn, void *arg)

{

uint8_t v = _Py_atomic_load_uint8(&flag->v);

for (;;) {

if (v == _Py_UNLOCKED) {

if (!_Py_atomic_compare_exchange_uint8(&flag->v, &v, _Py_LOCKED)) {

continue;

}

int res = fn(arg);

return unlock_once(flag, res);

}

if (v == _Py_ONCE_INITIALIZED) {

return 0;

}

// The once flag is initializing (locked).

assert((v & _Py_LOCKED));

if (!(v & _Py_HAS_PARKED)) {

// We are the first waiter. Set the _Py_HAS_PARKED flag.

uint8_t new_value = v | _Py_HAS_PARKED;

if (!_Py_atomic_compare_exchange_uint8(&flag->v, &v, new_value)) {

continue;

}

v = new_value;

}

// Wait for initialization to finish.

_PyParkingLot_Park(&flag->v, &v, sizeof(v), -1, NULL, 1);

v = _Py_atomic_load_uint8(&flag->v);

}

static int

recursive_mutex_is_owned_by(_PyRecursiveMutex *m, PyThread_ident_t tid)

{

return _Py_atomic_load_ullong_relaxed(&m->thread) == tid;

}

int

_PyRecursiveMutex_IsLockedByCurrentThread(_PyRecursiveMutex *m)

{

return recursive_mutex_is_owned_by(m, PyThread_get_thread_ident_ex());

}

void

_PyRecursiveMutex_Lock(_PyRecursiveMutex *m)

{

PyThread_ident_t thread = PyThread_get_thread_ident_ex();

if (recursive_mutex_is_owned_by(m, thread)) {

m->level++;

return;

}

PyMutex_Lock(&m->mutex);

_Py_atomic_store_ullong_relaxed(&m->thread, thread);

assert(m->level == 0);

}

PyLockStatus

_PyRecursiveMutex_LockTimed(_PyRecursiveMutex *m, PyTime_t timeout, _PyLockFlags flags)

{

PyThread_ident_t thread = PyThread_get_thread_ident_ex();

if (recursive_mutex_is_owned_by(m, thread)) {

m->level++;

return PY_LOCK_ACQUIRED;

}

PyLockStatus s = _PyMutex_LockTimed(&m->mutex, timeout, flags);

if (s == PY_LOCK_ACQUIRED) {

_Py_atomic_store_ullong_relaxed(&m->thread, thread);

assert(m->level == 0);

}

return s;

}

void

_PyRecursiveMutex_Unlock(_PyRecursiveMutex *m)

{

if (_PyRecursiveMutex_TryUnlock(m) < 0) {

Py_FatalError("unlocking a recursive mutex that is not "

"owned by the current thread");

}

int

_PyRecursiveMutex_TryUnlock(_PyRecursiveMutex *m)

{

PyThread_ident_t thread = PyThread_get_thread_ident_ex();

if (!recursive_mutex_is_owned_by(m, thread)) {

return -1;

}

if (m->level > 0) {

m->level--;

return 0;

}

assert(m->level == 0);

_Py_atomic_store_ullong_relaxed(&m->thread, 0);

PyMutex_Unlock(&m->mutex);

return 0;

}

#define _Py_WRITE_LOCKED 1

#define _PyRWMutex_READER_SHIFT 2

#define _Py_RWMUTEX_MAX_READERS (UINTPTR_MAX >> _PyRWMutex_READER_SHIFT)

static uintptr_t

rwmutex_set_parked_and_wait(_PyRWMutex *rwmutex, uintptr_t bits)

{

// Set _Py_HAS_PARKED and wait until we are woken up.

if ((bits & _Py_HAS_PARKED) == 0) {

uintptr_t newval = bits | _Py_HAS_PARKED;

if (!_Py_atomic_compare_exchange_uintptr(&rwmutex->bits,

&bits, newval)) {

return bits;

}

bits = newval;

}

_PyParkingLot_Park(&rwmutex->bits, &bits, sizeof(bits), -1, NULL, 1);

return _Py_atomic_load_uintptr_relaxed(&rwmutex->bits);

}

// The number of readers holding the lock

static uintptr_t

rwmutex_reader_count(uintptr_t bits)

{

return bits >> _PyRWMutex_READER_SHIFT;

}

void

_PyRWMutex_RLock(_PyRWMutex *rwmutex)

{

uintptr_t bits = _Py_atomic_load_uintptr_relaxed(&rwmutex->bits);

for (;;) {

if ((bits & _Py_WRITE_LOCKED)) {

// A writer already holds the lock.

bits = rwmutex_set_parked_and_wait(rwmutex, bits);

continue;

}

else if ((bits & _Py_HAS_PARKED)) {

// Reader(s) hold the lock (or just gave up the lock), but there is

// at least one waiting writer. We can't grab the lock because we

// don't want to starve the writer. Instead, we park ourselves and

// wait for the writer to eventually wake us up.

bits = rwmutex_set_parked_and_wait(rwmutex, bits);

continue;

}

else {

// The lock is unlocked or read-locked. Try to grab it.

assert(rwmutex_reader_count(bits) < _Py_RWMUTEX_MAX_READERS);

uintptr_t newval = bits + (1 << _PyRWMutex_READER_SHIFT);

if (!_Py_atomic_compare_exchange_uintptr(&rwmutex->bits,

&bits, newval)) {

continue;

}

return;

}

void

_PyRWMutex_RUnlock(_PyRWMutex *rwmutex)

{

uintptr_t bits = _Py_atomic_add_uintptr(&rwmutex->bits, -(1 << _PyRWMutex_READER_SHIFT));

assert(rwmutex_reader_count(bits) > 0 && "lock was not read-locked");

bits -= (1 << _PyRWMutex_READER_SHIFT);

if (rwmutex_reader_count(bits) == 0 && (bits & _Py_HAS_PARKED)) {

_PyParkingLot_UnparkAll(&rwmutex->bits);

return;

}

void

_PyRWMutex_Lock(_PyRWMutex *rwmutex)

{

uintptr_t bits = _Py_atomic_load_uintptr_relaxed(&rwmutex->bits);

for (;;) {

// If there are no active readers and it's not already write-locked,

// then we can grab the lock.

if ((bits & ~_Py_HAS_PARKED) == 0) {

if (!_Py_atomic_compare_exchange_uintptr(&rwmutex->bits,

&bits,

bits | _Py_WRITE_LOCKED)) {

continue;

}

return;

}

// Otherwise, we have to wait.

bits = rwmutex_set_parked_and_wait(rwmutex, bits);

}

void

_PyRWMutex_Unlock(_PyRWMutex *rwmutex)

{

uintptr_t old_bits = _Py_atomic_exchange_uintptr(&rwmutex->bits, 0);

assert((old_bits & _Py_WRITE_LOCKED) && "lock was not write-locked");

assert(rwmutex_reader_count(old_bits) == 0 && "lock was read-locked");

if ((old_bits & _Py_HAS_PARKED) != 0) {

_PyParkingLot_UnparkAll(&rwmutex->bits);

}

#undef PyMutex_Lock

void

PyMutex_Lock(PyMutex *m)

{

_PyMutex_LockTimed(m, -1, _PY_LOCK_DETACH);

}

#undef PyMutex_Unlock

void

PyMutex_Unlock(PyMutex *m)

{

if (_PyMutex_TryUnlock(m) < 0) {

Py_FatalError("unlocking mutex that is not locked");

}

#undef PyMutex_IsLocked

int

PyMutex_IsLocked(PyMutex *m)

{

return _PyMutex_IsLocked(m);

}

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