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LibPthread: Reimplement semaphores
This implementation does not use locking or condition variables internally; it's purely based on atomics and futexes. Notably, concurrent sem_wait() and sem_post() calls can run *completely in parallel* without slowing each other down, as long as there are empty slots for them all to succeed without blocking. Additionally, sem_wait() never executes an atomic operation with release ordering, and sem_post() never executes an atomic operation with acquire ordering (unless you count the syscall). This means the compiler and the hardware are free to reorder code *into* the critical section.
This commit is contained in:
Sergey Bugaev
Andreas Kling
parent
00d8dbe739
commit
690141ff8b
Notes:
sideshowbarker
2024-07-18 10:21:27 +09:00
Author: https://github.com/bugaevc Commit: https://github.com/SerenityOS/serenity/commit/690141ff8b3 Pull-request: https://github.com/SerenityOS/serenity/pull/8455 Issue: https://github.com/SerenityOS/serenity/issues/8432 Reviewed-by: https://github.com/alimpfard
@ -1,12 +1,26 @@
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/*
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* Copyright (c) 2021, the SerenityOS developers.
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* Copyright (c) 2021, Gunnar Beutner <gbeutner@serenityos.org>
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* Copyright (c) 2021, Sergey Bugaev <bugaevc@serenityos.org>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#include <AK/Assertions.h>
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#include <AK/Atomic.h>
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#include <AK/Types.h>
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#include <errno.h>
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#include <semaphore.h>
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#include <serenity.h>
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// Whether sem_wait() or sem_post() is responsible for waking any sleeping
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// threads.
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static constexpr u32 POST_WAKES = 1 << 31;
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sem_t* sem_open(const char*, int, ...)
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{
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errno = ENOSYS;
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return nullptr;
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}
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int sem_close(sem_t*)
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{
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@ -14,42 +28,10 @@ int sem_close(sem_t*)
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return -1;
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}
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int sem_destroy(sem_t* sem)
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int sem_unlink(const char*)
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{
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auto rc = pthread_mutex_destroy(&sem->mtx);
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if (rc != 0) {
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errno = rc;
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return -1;
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}
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rc = pthread_cond_destroy(&sem->cv);
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if (rc != 0) {
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errno = rc;
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return -1;
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}
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return 0;
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}
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int sem_getvalue(sem_t* sem, int* sval)
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{
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auto rc = pthread_mutex_trylock(&sem->mtx);
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if (rc == EBUSY) {
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*sval = 0;
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return 0;
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}
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if (rc != 0) {
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errno = rc;
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return -1;
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}
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*sval = sem->value;
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pthread_mutex_unlock(&sem->mtx);
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return 0;
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errno = ENOSYS;
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return -1;
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}
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int sem_init(sem_t* sem, int shared, unsigned int value)
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@ -64,116 +46,110 @@ int sem_init(sem_t* sem, int shared, unsigned int value)
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return -1;
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}
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auto rc = pthread_mutex_init(&sem->mtx, nullptr);
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if (rc != 0) {
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errno = rc;
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return -1;
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}
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rc = pthread_cond_init(&sem->cv, nullptr);
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if (rc != 0) {
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errno = rc;
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return -1;
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}
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sem->value = value;
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return 0;
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}
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sem_t* sem_open(const char*, int, ...)
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int sem_destroy(sem_t*)
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{
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errno = ENOSYS;
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return nullptr;
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return 0;
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}
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int sem_getvalue(sem_t* sem, int* sval)
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{
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u32 value = AK::atomic_load(&sem->value, AK::memory_order_relaxed);
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*sval = value & ~POST_WAKES;
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return 0;
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}
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int sem_post(sem_t* sem)
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{
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auto rc = pthread_mutex_lock(&sem->mtx);
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if (rc != 0) {
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errno = rc;
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return -1;
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}
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if (sem->value == SEM_VALUE_MAX) {
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pthread_mutex_unlock(&sem->mtx);
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errno = EOVERFLOW;
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return -1;
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}
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sem->value++;
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rc = pthread_cond_signal(&sem->cv);
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if (rc != 0) {
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pthread_mutex_unlock(&sem->mtx);
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errno = rc;
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return -1;
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}
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rc = pthread_mutex_unlock(&sem->mtx);
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if (rc != 0) {
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errno = rc;
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return -1;
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}
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u32 value = AK::atomic_fetch_add(&sem->value, 1u, AK::memory_order_release);
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// Fast path: no need to wake.
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if (!(value & POST_WAKES)) [[likely]]
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return 0;
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// Pass the responsibility for waking more threads if more slots become
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// available later to sem_wait() in the thread we're about to wake, as
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// opposed to further sem_post() calls that free up those slots.
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value = AK::atomic_fetch_and(&sem->value, ~POST_WAKES, AK::memory_order_relaxed);
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// Check if another sem_post() call has handled it already.
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if (!(value & POST_WAKES)) [[likely]]
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return 0;
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int rc = futex_wake(&sem->value, 1);
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VERIFY(rc == 0);
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return 0;
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}
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int sem_trywait(sem_t* sem)
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{
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auto rc = pthread_mutex_lock(&sem->mtx);
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if (rc != 0) {
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errno = rc;
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return -1;
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}
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if (sem->value == 0) {
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pthread_mutex_unlock(&sem->mtx);
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errno = EAGAIN;
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return -1;
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}
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sem->value--;
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rc = pthread_mutex_unlock(&sem->mtx);
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if (rc != 0) {
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errno = rc;
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return -1;
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}
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return 0;
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}
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int sem_unlink(const char*)
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{
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errno = ENOSYS;
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return -1;
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u32 value = AK::atomic_load(&sem->value, AK::memory_order_relaxed);
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u32 count = value & ~POST_WAKES;
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if (count == 0)
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return EAGAIN;
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// Decrement the count without touching the flag.
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u32 desired = (count - 1) | (value & POST_WAKES);
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bool exchanged = AK::atomic_compare_exchange_strong(&sem->value, value, desired, AK::memory_order_acquire);
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if (exchanged) [[likely]]
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return 0;
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else
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return EAGAIN;
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}
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int sem_wait(sem_t* sem)
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{
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auto rc = pthread_mutex_lock(&sem->mtx);
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if (rc != 0) {
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errno = rc;
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return -1;
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}
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while (sem->value == 0) {
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rc = pthread_cond_wait(&sem->cv, &sem->mtx);
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if (rc != 0) {
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pthread_mutex_unlock(&sem->mtx);
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errno = rc;
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return -1;
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}
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}
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sem->value--;
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rc = pthread_mutex_unlock(&sem->mtx);
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if (rc != 0) {
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errno = rc;
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return -1;
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}
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return 0;
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return sem_timedwait(sem, nullptr);
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}
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int sem_timedwait(sem_t* sem, const struct timespec* abstime)
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{
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u32 value = AK::atomic_load(&sem->value, AK::memory_order_relaxed);
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bool responsible_for_waking = false;
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while (true) {
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u32 count = value & ~POST_WAKES;
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if (count > 0) [[likely]] {
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// It looks like there are some free slots.
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u32 whether_post_wakes = value & POST_WAKES;
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bool going_to_wake = false;
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if (responsible_for_waking && !whether_post_wakes) {
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// If we have ourselves been woken up previously, and the
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// POST_WAKES flag is not set, that means some more slots might
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// be available now, and it's us who has to wake up additional
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// threads.
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if (count > 1) [[unlikely]]
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going_to_wake = true;
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// Pass the responsibility for waking up further threads back to
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// sem_post() calls. In particular, we don't want the threads
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// we're about to wake to try to wake anyone else.
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whether_post_wakes = POST_WAKES;
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}
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// Now, try to commit this.
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u32 desired = (count - 1) | whether_post_wakes;
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bool exchanged = AK::atomic_compare_exchange_strong(&sem->value, value, desired, AK::memory_order_acquire);
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if (!exchanged) [[unlikely]]
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// Re-evaluate.
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continue;
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if (going_to_wake) [[unlikely]] {
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int rc = futex_wake(&sem->value, count - 1);
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VERIFY(rc >= 0);
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}
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return 0;
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}
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// We're probably going to sleep, so attempt to set the flag. We do not
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// commit to sleeping yet, though, as setting the flag may fail and
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// cause us to reevaluate what we're doing.
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if (value == 0) {
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bool exchanged = AK::atomic_compare_exchange_strong(&sem->value, value, POST_WAKES, AK::memory_order_relaxed);
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if (!exchanged) [[unlikely]]
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// Re-evaluate.
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continue;
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value = POST_WAKES;
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}
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// At this point, we're committed to sleeping.
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responsible_for_waking = true;
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futex_wait(&sem->value, value, abstime, CLOCK_REALTIME);
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// This is the state we will probably see upon being waked:
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value = 1;
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}
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}
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@ -14,9 +14,7 @@
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__BEGIN_DECLS
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typedef struct {
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pthread_mutex_t mtx;
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pthread_cond_t cv;
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int value;
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uint32_t value;
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} sem_t;
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int sem_close(sem_t*);
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@ -28,6 +26,7 @@ int sem_post(sem_t*);
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int sem_trywait(sem_t*);
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int sem_unlink(const char*);
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int sem_wait(sem_t*);
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int sem_timedwait(sem_t*, const struct timespec* abstime);
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#define SEM_VALUE_MAX INT_MAX
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