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Copy pathlock.c
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297 lines (261 loc) · 7.77 KB
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// Lock implementation
#include "Python.h"
#include "pycore_lock.h"
#include "pycore_parking_lot.h"
#include "pycore_semaphore.h"
#ifdef MS_WINDOWS
#define WIN32_LEAN_AND_MEAN
#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_NOGIL
static const int MAX_SPIN_COUNT = 40;
#else
static const int MAX_SPIN_COUNT = 0;
#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;
};
static void
_Py_yield(void)
{
#ifdef MS_WINDOWS
SwitchToThread();
#elif defined(HAVE_SCHED_H)
sched_yield();
#endif
}
void
_PyMutex_LockSlow(PyMutex *m)
{
_PyMutex_LockTimed(m, -1, _PY_LOCK_DETACH);
}
PyLockStatus
_PyMutex_LockTimed(PyMutex *m, _PyTime_t timeout, _PyLockFlags flags)
{
uint8_t v = _Py_atomic_load_uint8_relaxed(&m->v);
if ((v & _Py_LOCKED) == 0) {
if (_Py_atomic_compare_exchange_uint8(&m->v, &v, v|_Py_LOCKED)) {
return PY_LOCK_ACQUIRED;
}
}
else if (timeout == 0) {
return PY_LOCK_FAILURE;
}
_PyTime_t now = _PyTime_GetMonotonicClock();
_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;
for (;;) {
if ((v & _Py_LOCKED) == 0) {
// The lock is unlocked. Try to grab it.
if (_Py_atomic_compare_exchange_uint8(&m->v, &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++;
continue;
}
if (timeout == 0) {
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->v, &v, newv)) {
continue;
}
}
int ret = _PyParkingLot_Park(&m->v, &newv, sizeof(newv), timeout,
&entry, (flags & _PY_LOCK_DETACH) != 0);
if (ret == Py_PARK_OK) {
if (entry.handed_off) {
// We own the lock now.
assert(_Py_atomic_load_uint8_relaxed(&m->v) & _Py_LOCKED);
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_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->v);
}
}
static void
mutex_unpark(PyMutex *m, struct mutex_entry *entry, int has_more_waiters)
{
uint8_t v = 0;
if (entry) {
_PyTime_t now = _PyTime_GetMonotonicClock();
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->v, v);
}
int
_PyMutex_TryUnlock(PyMutex *m)
{
uint8_t v = _Py_atomic_load_uint8(&m->v);
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->v, (_Py_unpark_fn_t *)mutex_unpark, m);
return 0;
}
else if (_Py_atomic_compare_exchange_uint8(&m->v, &v, _Py_UNLOCKED)) {
// fast-path: no waiters
return 0;
}
}
}
void
_PyMutex_UnlockSlow(PyMutex *m)
{
if (_PyMutex_TryUnlock(m) < 0) {
Py_FatalError("unlocking mutex that is not locked");
}
}
// _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.
_PySemaphore_Wait(&waiter.sema, -1, /*detach=*/0);
}
_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;
}
}
}
}
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))
;
}
int
PyEvent_WaitTimed(PyEvent *evt, _PyTime_t timeout_ns)
{
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, 1);
return _Py_atomic_load_uint8(&evt->v) == _Py_LOCKED;
}
}