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- /*
- * Copyright 2015 Google Inc.
- *
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file.
- */
- #ifndef SkSemaphore_DEFINED
- #define SkSemaphore_DEFINED
- #include "include/core/SkTypes.h"
- #include "include/private/SkOnce.h"
- #include <atomic>
- class SkSemaphore {
- public:
- constexpr SkSemaphore(int count = 0) : fCount(count), fOSSemaphore(nullptr) {}
- // Cleanup the underlying OS semaphore.
- ~SkSemaphore();
- // Increment the counter n times.
- // Generally it's better to call signal(n) instead of signal() n times.
- void signal(int n = 1);
- // Decrement the counter by 1,
- // then if the counter is < 0, sleep this thread until the counter is >= 0.
- void wait();
- // If the counter is positive, decrement it by 1 and return true, otherwise return false.
- bool try_wait();
- private:
- // This implementation follows the general strategy of
- // 'A Lightweight Semaphore with Partial Spinning'
- // found here
- // http://preshing.com/20150316/semaphores-are-surprisingly-versatile/
- // That article (and entire blog) are very much worth reading.
- //
- // We wrap an OS-provided semaphore with a user-space atomic counter that
- // lets us avoid interacting with the OS semaphore unless strictly required:
- // moving the count from >=0 to <0 or vice-versa, i.e. sleeping or waking threads.
- struct OSSemaphore;
- void osSignal(int n);
- void osWait();
- std::atomic<int> fCount;
- SkOnce fOSSemaphoreOnce;
- OSSemaphore* fOSSemaphore;
- };
- inline void SkSemaphore::signal(int n) {
- int prev = fCount.fetch_add(n, std::memory_order_release);
- // We only want to call the OS semaphore when our logical count crosses
- // from <0 to >=0 (when we need to wake sleeping threads).
- //
- // This is easiest to think about with specific examples of prev and n.
- // If n == 5 and prev == -3, there are 3 threads sleeping and we signal
- // SkTMin(-(-3), 5) == 3 times on the OS semaphore, leaving the count at 2.
- //
- // If prev >= 0, no threads are waiting, SkTMin(-prev, n) is always <= 0,
- // so we don't call the OS semaphore, leaving the count at (prev + n).
- int toSignal = SkTMin(-prev, n);
- if (toSignal > 0) {
- this->osSignal(toSignal);
- }
- }
- inline void SkSemaphore::wait() {
- // Since this fetches the value before the subtract, zero and below means that there are no
- // resources left, so the thread needs to wait.
- if (fCount.fetch_sub(1, std::memory_order_acquire) <= 0) {
- this->osWait();
- }
- }
- #endif//SkSemaphore_DEFINED
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