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cpython/Python/parking_lot.c

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#include "Python.h"
#include "pycore_llist.h"
#include "pycore_lock.h" // _PyRawMutex
#include "pycore_parking_lot.h"
#include "pycore_pyerrors.h" // _Py_FatalErrorFormat
#include "pycore_pystate.h" // _PyThreadState_GET
#include "pycore_semaphore.h" // _PySemaphore
#include <stdbool.h>
typedef struct {
// The mutex protects the waiter queue and the num_waiters counter.
_PyRawMutex mutex;
// Linked list of `struct wait_entry` waiters in this bucket.
struct llist_node root;
size_t num_waiters;
} Bucket;
struct wait_entry {
void *park_arg;
uintptr_t addr;
_PySemaphore sema;
struct llist_node node;
bool is_unparking;
};
// Prime number to avoid correlations with memory addresses.
// We want this to be roughly proportional to the number of CPU cores
// to minimize contention on the bucket locks, but not too big to avoid
// wasting memory. The exact choice does not matter much.
#define NUM_BUCKETS 257
#define BUCKET_INIT(b, i) [i] = { .root = LLIST_INIT(b[i].root) }
#define BUCKET_INIT_2(b, i) BUCKET_INIT(b, i), BUCKET_INIT(b, i+1)
#define BUCKET_INIT_4(b, i) BUCKET_INIT_2(b, i), BUCKET_INIT_2(b, i+2)
#define BUCKET_INIT_8(b, i) BUCKET_INIT_4(b, i), BUCKET_INIT_4(b, i+4)
#define BUCKET_INIT_16(b, i) BUCKET_INIT_8(b, i), BUCKET_INIT_8(b, i+8)
#define BUCKET_INIT_32(b, i) BUCKET_INIT_16(b, i), BUCKET_INIT_16(b, i+16)
#define BUCKET_INIT_64(b, i) BUCKET_INIT_32(b, i), BUCKET_INIT_32(b, i+32)
#define BUCKET_INIT_128(b, i) BUCKET_INIT_64(b, i), BUCKET_INIT_64(b, i+64)
#define BUCKET_INIT_256(b, i) BUCKET_INIT_128(b, i), BUCKET_INIT_128(b, i+128)
// Table of waiters (hashed by address)
static Bucket buckets[NUM_BUCKETS] = {
BUCKET_INIT_256(buckets, 0),
BUCKET_INIT(buckets, 256),
};
void
_PySemaphore_Init(_PySemaphore *sema)
{
#if defined(MS_WINDOWS)
sema->platform_sem = CreateSemaphore(
NULL, // attributes
0, // initial count
10, // maximum count
NULL // unnamed
);
if (!sema->platform_sem) {
Py_FatalError("parking_lot: CreateSemaphore failed");
}
#elif defined(_Py_USE_SEMAPHORES)
if (sem_init(&sema->platform_sem, /*pshared=*/0, /*value=*/0) < 0) {
Py_FatalError("parking_lot: sem_init failed");
}
#else
if (pthread_mutex_init(&sema->mutex, NULL) != 0) {
Py_FatalError("parking_lot: pthread_mutex_init failed");
}
if (pthread_cond_init(&sema->cond, NULL)) {
Py_FatalError("parking_lot: pthread_cond_init failed");
}
sema->counter = 0;
#endif
}
void
_PySemaphore_Destroy(_PySemaphore *sema)
{
#if defined(MS_WINDOWS)
CloseHandle(sema->platform_sem);
#elif defined(_Py_USE_SEMAPHORES)
sem_destroy(&sema->platform_sem);
#else
pthread_mutex_destroy(&sema->mutex);
pthread_cond_destroy(&sema->cond);
#endif
}
static int
_PySemaphore_PlatformWait(_PySemaphore *sema, _PyTime_t timeout)
{
int res;
#if defined(MS_WINDOWS)
DWORD wait;
DWORD millis = 0;
if (timeout < 0) {
millis = INFINITE;
}
else {
millis = (DWORD) (timeout / 1000000);
}
wait = WaitForSingleObjectEx(sema->platform_sem, millis, FALSE);
if (wait == WAIT_OBJECT_0) {
res = Py_PARK_OK;
}
else if (wait == WAIT_TIMEOUT) {
res = Py_PARK_TIMEOUT;
}
else {
res = Py_PARK_INTR;
}
#elif defined(_Py_USE_SEMAPHORES)
int err;
if (timeout >= 0) {
struct timespec ts;
#if defined(CLOCK_MONOTONIC) && defined(HAVE_SEM_CLOCKWAIT)
_PyTime_t deadline = _PyTime_Add(_PyTime_GetMonotonicClock(), timeout);
_PyTime_AsTimespec_clamp(deadline, &ts);
err = sem_clockwait(&sema->platform_sem, CLOCK_MONOTONIC, &ts);
#else
_PyTime_t deadline = _PyTime_Add(_PyTime_GetSystemClock(), timeout);
_PyTime_AsTimespec_clamp(deadline, &ts);
err = sem_timedwait(&sema->platform_sem, &ts);
#endif
}
else {
err = sem_wait(&sema->platform_sem);
}
if (err == -1) {
err = errno;
if (err == EINTR) {
res = Py_PARK_INTR;
}
else if (err == ETIMEDOUT) {
res = Py_PARK_TIMEOUT;
}
else {
_Py_FatalErrorFormat(__func__,
"unexpected error from semaphore: %d",
err);
}
}
else {
res = Py_PARK_OK;
}
#else
pthread_mutex_lock(&sema->mutex);
int err = 0;
if (sema->counter == 0) {
if (timeout >= 0) {
struct timespec ts;
_PyTime_t deadline = _PyTime_Add(_PyTime_GetSystemClock(), timeout);
_PyTime_AsTimespec_clamp(deadline, &ts);
err = pthread_cond_timedwait(&sema->cond, &sema->mutex, &ts);
}
else {
err = pthread_cond_wait(&sema->cond, &sema->mutex);
}
}
if (sema->counter > 0) {
sema->counter--;
res = Py_PARK_OK;
}
else if (err) {
res = Py_PARK_TIMEOUT;
}
else {
res = Py_PARK_INTR;
}
pthread_mutex_unlock(&sema->mutex);
#endif
return res;
}
int
_PySemaphore_Wait(_PySemaphore *sema, _PyTime_t timeout, int detach)
{
PyThreadState *tstate = NULL;
if (detach) {
tstate = _PyThreadState_GET();
if (tstate) {
PyEval_ReleaseThread(tstate);
}
}
int res = _PySemaphore_PlatformWait(sema, timeout);
if (detach && tstate) {
PyEval_AcquireThread(tstate);
}
return res;
}
void
_PySemaphore_Wakeup(_PySemaphore *sema)
{
#if defined(MS_WINDOWS)
if (!ReleaseSemaphore(sema->platform_sem, 1, NULL)) {
Py_FatalError("parking_lot: ReleaseSemaphore failed");
}
#elif defined(_Py_USE_SEMAPHORES)
int err = sem_post(&sema->platform_sem);
if (err != 0) {
Py_FatalError("parking_lot: sem_post failed");
}
#else
pthread_mutex_lock(&sema->mutex);
sema->counter++;
pthread_cond_signal(&sema->cond);
pthread_mutex_unlock(&sema->mutex);
#endif
}
static void
enqueue(Bucket *bucket, const void *address, struct wait_entry *wait)
{
llist_insert_tail(&bucket->root, &wait->node);
++bucket->num_waiters;
}
static struct wait_entry *
dequeue(Bucket *bucket, const void *address)
{
// find the first waiter that is waiting on `address`
struct llist_node *root = &bucket->root;
struct llist_node *node;
llist_for_each(node, root) {
struct wait_entry *wait = llist_data(node, struct wait_entry, node);
if (wait->addr == (uintptr_t)address) {
llist_remove(node);
--bucket->num_waiters;
return wait;
}
}
return NULL;
}
static void
dequeue_all(Bucket *bucket, const void *address, struct llist_node *dst)
{
// remove and append all matching waiters to dst
struct llist_node *root = &bucket->root;
struct llist_node *node;
llist_for_each_safe(node, root) {
struct wait_entry *wait = llist_data(node, struct wait_entry, node);
if (wait->addr == (uintptr_t)address) {
llist_remove(node);
llist_insert_tail(dst, node);
--bucket->num_waiters;
}
}
}
// Checks that `*addr == *expected` (only works for 1, 2, 4, or 8 bytes)
static int
atomic_memcmp(const void *addr, const void *expected, size_t addr_size)
{
switch (addr_size) {
case 1: return _Py_atomic_load_uint8(addr) == *(const uint8_t *)expected;
case 2: return _Py_atomic_load_uint16(addr) == *(const uint16_t *)expected;
case 4: return _Py_atomic_load_uint32(addr) == *(const uint32_t *)expected;
case 8: return _Py_atomic_load_uint64(addr) == *(const uint64_t *)expected;
default: Py_UNREACHABLE();
}
}
int
_PyParkingLot_Park(const void *addr, const void *expected, size_t size,
_PyTime_t timeout_ns, void *park_arg, int detach)
{
struct wait_entry wait = {
.park_arg = park_arg,
.addr = (uintptr_t)addr,
.is_unparking = false,
};
Bucket *bucket = &buckets[((uintptr_t)addr) % NUM_BUCKETS];
_PyRawMutex_Lock(&bucket->mutex);
if (!atomic_memcmp(addr, expected, size)) {
_PyRawMutex_Unlock(&bucket->mutex);
return Py_PARK_AGAIN;
}
_PySemaphore_Init(&wait.sema);
enqueue(bucket, addr, &wait);
_PyRawMutex_Unlock(&bucket->mutex);
int res = _PySemaphore_Wait(&wait.sema, timeout_ns, detach);
if (res == Py_PARK_OK) {
goto done;
}
// timeout or interrupt
_PyRawMutex_Lock(&bucket->mutex);
if (wait.is_unparking) {
_PyRawMutex_Unlock(&bucket->mutex);
// Another thread has started to unpark us. Wait until we process the
// wakeup signal.
do {
res = _PySemaphore_Wait(&wait.sema, -1, detach);
} while (res != Py_PARK_OK);
goto done;
}
else {
llist_remove(&wait.node);
--bucket->num_waiters;
}
_PyRawMutex_Unlock(&bucket->mutex);
done:
_PySemaphore_Destroy(&wait.sema);
return res;
}
void
_PyParkingLot_Unpark(const void *addr, _Py_unpark_fn_t *fn, void *arg)
{
Bucket *bucket = &buckets[((uintptr_t)addr) % NUM_BUCKETS];
// Find the first waiter that is waiting on `addr`
_PyRawMutex_Lock(&bucket->mutex);
struct wait_entry *waiter = dequeue(bucket, addr);
if (waiter) {
waiter->is_unparking = true;
int has_more_waiters = (bucket->num_waiters > 0);
fn(arg, waiter->park_arg, has_more_waiters);
}
else {
fn(arg, NULL, 0);
}
_PyRawMutex_Unlock(&bucket->mutex);
if (waiter) {
// Wakeup the waiter outside of the bucket lock
_PySemaphore_Wakeup(&waiter->sema);
}
}
void
_PyParkingLot_UnparkAll(const void *addr)
{
struct llist_node head = LLIST_INIT(head);
Bucket *bucket = &buckets[((uintptr_t)addr) % NUM_BUCKETS];
_PyRawMutex_Lock(&bucket->mutex);
dequeue_all(bucket, addr, &head);
_PyRawMutex_Unlock(&bucket->mutex);
struct llist_node *node;
llist_for_each_safe(node, &head) {
struct wait_entry *waiter = llist_data(node, struct wait_entry, node);
llist_remove(node);
_PySemaphore_Wakeup(&waiter->sema);
}
}
void
_PyParkingLot_AfterFork(void)
{
// After a fork only one thread remains. That thread cannot be blocked
// so all entries in the parking lot are for dead threads.
memset(buckets, 0, sizeof(buckets));
for (Py_ssize_t i = 0; i < NUM_BUCKETS; i++) {
llist_init(&buckets[i].root);
}
}
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