Revert of Modernize SkMutex and SkSemaphore.

src/core/SkSemaphore.cpp

Index: src/core/SkSemaphore.cpp
diff --git a/src/core/SkSemaphore.cpp b/src/core/SkSemaphore.cpp
index 0646b152e884834bacbe1506e13c30627f451c98..da422e282f6ea8a367c187c9e81a02b347e8ad4a 100644
--- a/src/core/SkSemaphore.cpp
+++ b/src/core/SkSemaphore.cpp
@@ -9,7 +9,7 @@
 
 #if defined(SK_BUILD_FOR_MAC) || defined(SK_BUILD_FOR_IOS)
     #include <mach/mach.h>
-    struct SkSemaphore::OSSemaphore {
+    struct SkBaseSemaphore::OSSemaphore {
         semaphore_t fSemaphore;
 
         OSSemaphore()  {
@@ -21,7 +21,7 @@
         void wait() { semaphore_wait(fSemaphore); }
     };
 #elif defined(SK_BUILD_FOR_WIN32)
-    struct SkSemaphore::OSSemaphore {
+    struct SkBaseSemaphore::OSSemaphore {
         HANDLE fSemaphore;
 
         OSSemaphore()  {
@@ -41,7 +41,7 @@
     // It's important we test for Mach before this.  This code will compile but not work there.
     #include <errno.h>
     #include <semaphore.h>
-    struct SkSemaphore::OSSemaphore {
+    struct SkBaseSemaphore::OSSemaphore {
         sem_t fSemaphore;
 
         OSSemaphore()  { sem_init(&fSemaphore, 0/*cross process?*/, 0/*initial count*/); }
@@ -57,16 +57,43 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
-void SkSemaphore::osSignal(int n) {
-    fOSSemaphoreOnce([this] { fOSSemaphore = new OSSemaphore; });
-    fOSSemaphore->signal(n);
+void SkBaseSemaphore::signal(int n) {
+    SkASSERT(n >= 0);
+
+    // 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 prev = sk_atomic_fetch_add(&fCount, n, sk_memory_order_release);
+    int toSignal = SkTMin(-prev, n);
+    if (toSignal > 0) {
+        this->osSignal(toSignal);
+    }
 }
 
-void SkSemaphore::osWait() {
-    fOSSemaphoreOnce([this] { fOSSemaphore = new OSSemaphore; });
-    fOSSemaphore->wait();
+static SkBaseSemaphore::OSSemaphore* semaphore(SkBaseSemaphore* semaphore) {
+    return semaphore->fOSSemaphore.get([](){ return new SkBaseSemaphore::OSSemaphore(); });
 }
 
-SkSemaphore::~SkSemaphore() {
-    delete fOSSemaphore;
+void SkBaseSemaphore::osSignal(int n) { semaphore(this)->signal(n); }
+
+void SkBaseSemaphore::osWait() { semaphore(this)->wait(); }
+
+void SkBaseSemaphore::deleteSemaphore() {
+    delete (OSSemaphore*) fOSSemaphore;
 }
+
+///////////////////////////////////////////////////////////////////////////////
+
+SkSemaphore::SkSemaphore(){ fBaseSemaphore = {0, {0}}; }
+
+SkSemaphore::~SkSemaphore() { fBaseSemaphore.deleteSemaphore(); }
+
+void SkSemaphore::wait() { fBaseSemaphore.wait(); }
+
+void SkSemaphore::signal(int n) {fBaseSemaphore.signal(n); }