diff options
author | mtklein <mtklein@chromium.org> | 2016-05-05 10:57:37 -0700 |
---|---|---|
committer | Commit bot <commit-bot@chromium.org> | 2016-05-05 10:57:37 -0700 |
commit | 42846ed43767740993f6eb1cdb62f208713304c9 (patch) | |
tree | a5e301c6bc52710b2a92d714e1087b8fe8996dfd /include/private/SkSemaphore.h | |
parent | d6f25bf0ef353da39859fb4173d2cf60d52277e4 (diff) |
Modernize SkMutex and SkSemaphore.
- use <atomic>
- fuse SkMutex and SkBaseMutex
- fuse SkSemaphore and SkBaseSemaphore
Still TODO:
- replace SK_DECLARE_STATIC_MUTEX(name) with static SkMutex name
I just didn't want to bother fixing all that up until I know this CL sticks.
BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1947153002
No public API changes.
TBR=reed@google.com
Committed: https://skia.googlesource.com/skia/+/427c2819d9237d7d7729c59238036cfc73c072ea
Review-Url: https://codereview.chromium.org/1947153002
Diffstat (limited to 'include/private/SkSemaphore.h')
-rw-r--r-- | include/private/SkSemaphore.h | 99 |
1 files changed, 48 insertions, 51 deletions
diff --git a/include/private/SkSemaphore.h b/include/private/SkSemaphore.h index cb2f58da4a..3da2b99ab4 100644 --- a/include/private/SkSemaphore.h +++ b/include/private/SkSemaphore.h @@ -8,43 +8,27 @@ #ifndef SkSemaphore_DEFINED #define SkSemaphore_DEFINED +#include "../private/SkOnce.h" #include "SkTypes.h" -#include "../private/SkAtomics.h" -#include "../private/SkOncePtr.h" +#include <atomic> -struct SkBaseSemaphore { - - // Increment the counter by 1. - // This is a specialization for supporting SkMutex. - void signal() { - // Since this fetches the value before the add, 0 indicates that this thread is running and - // no threads are waiting, -1 and below means that threads are waiting, but only signal 1 - // thread to run. - if (sk_atomic_fetch_add(&fCount, 1, sk_memory_order_release) < 0) { - this->osSignal(1); - } - } +class SkBaseSemaphore { +public: + constexpr SkBaseSemaphore(int count = 0) + : fCount(count), fOSSemaphore(nullptr) {} - // Increment the counter N times. - // Generally it's better to call signal(N) instead of signal() N times. - void signal(int N); + // 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() { - // 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 (sk_atomic_fetch_sub(&fCount, 1, sk_memory_order_acquire) <= 0) { - this->osWait(); - } - } - - struct OSSemaphore; + void wait(); - void osSignal(int n); - void osWait(); - void deleteSemaphore(); + // SkBaseSemaphore has no destructor. Call this to clean it up. + void cleanup(); +private: // This implementation follows the general strategy of // 'A Lightweight Semaphore with Partial Spinning' // found here @@ -54,33 +38,46 @@ struct SkBaseSemaphore { // 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. - int fCount; - SkBaseOncePtr<OSSemaphore> fOSSemaphore; + struct OSSemaphore; + + void osSignal(int n); + void osWait(); + + std::atomic<int> fCount; + SkOnce fOSSemaphoreOnce; + OSSemaphore* fOSSemaphore; }; -/** - * SkSemaphore is a fast mostly-user-space semaphore. - * - * A semaphore is logically an atomic integer with a few special properties: - * - The integer always starts at 0. - * - You can only increment or decrement it, never read or write it. - * - Increment is spelled 'signal()'; decrement is spelled 'wait()'. - * - If a call to wait() decrements the counter to <= 0, - * the calling thread sleeps until another thread signal()s it back above 0. - */ -class SkSemaphore : SkNoncopyable { +class SkSemaphore : public SkBaseSemaphore { public: - // Initializes the counter to 0. - // (Though all current implementations could start from an arbitrary value.) - SkSemaphore(); - ~SkSemaphore(); + using SkBaseSemaphore::SkBaseSemaphore; + ~SkSemaphore() { this->cleanup(); } +}; - void wait(); +inline void SkBaseSemaphore::signal(int n) { + int prev = fCount.fetch_add(n, std::memory_order_release); - void signal(int n = 1); + // 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); + } +} -private: - SkBaseSemaphore fBaseSemaphore; -}; +inline void SkBaseSemaphore::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 |