Add and use SkSemaphore

src/core/SkTaskGroup.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
+#include "SkRunnable.h"
+#include "SkSemaphore.h"
+#include "SkSpinlock.h"
+#include "SkTDArray.h"
 #include "SkTaskGroup.h"
-
-#include "SkCondVar.h"
-#include "SkRunnable.h"
-#include "SkTDArray.h"
-#include "SkThread.h"
 #include "SkThreadUtils.h"
 
 #if defined(SK_BUILD_FOR_WIN32)
     static inline int num_cores() {
         SYSTEM_INFO sysinfo;
         GetSystemInfo(&sysinfo);
         return sysinfo.dwNumberOfProcessors;
     }
 #else
     #include <unistd.h>
@@ skipping 32 matching lines @@
     static void Wait(SkAtomic<int32_t>* pending) {
         if (!gGlobal) {  // If we have no threads, the work must already be done.
             SkASSERT(pending->load(sk_memory_order_relaxed) == 0);
             return;
         }
         // Acquire pairs with decrement release here or in Loop.
         while (pending->load(sk_memory_order_acquire) > 0) {
             // Lend a hand until our SkTaskGroup of interest is done.
             Work work;
             {
-                AutoLock lock(&gGlobal->fReady);
+                // We're stealing work opportunistically,
+                // so we never call fWorkAvailable.wait(), which could sleep us if there's no work.
+                // This means fWorkAvailable is only an upper bound on fWork.count().
+                AutoLock lock(&gGlobal->fWorkLock);
                 if (gGlobal->fWork.isEmpty()) {
                     // Someone has picked up all the work (including ours).  How nice of them!
                     // (They may still be working on it, so we can't assert *pending == 0 here.)
                     continue;
                 }
                 gGlobal->fWork.pop(&work);
             }
             // This Work isn't necessarily part of our SkTaskGroup of interest, but that's fine.
             // We threads gotta stick together.  We're always making forward progress.
             work.fn(work.arg);
             work.pending->fetch_add(-1, sk_memory_order_release);  // Pairs with load above.
         }
     }
 
 private:
     struct AutoLock {
-        AutoLock(SkCondVar* c) : fC(c) { fC->lock(); }
-        ~AutoLock() { fC->unlock(); }
+        AutoLock(SkSpinlock* lock) : fLock(lock) { fLock->acquire(); }
+        ~AutoLock() { fLock->release(); }
     private:
-        SkCondVar* fC;
+        SkSpinlock* fLock;
     };
 
     static void CallRunnable(void* arg) { static_cast<SkRunnable*>(arg)->run(); }
 
     struct Work {
         void (*fn)(void*);            // A function to call,
         void* arg;                    // its argument,
         SkAtomic<int32_t>* pending;   // then decrement pending afterwards.
     };
 
-    explicit ThreadPool(int threads) : fDraining(false) {
+    explicit ThreadPool(int threads) {
         if (threads == -1) {
             threads = num_cores();
         }
         for (int i = 0; i < threads; i++) {
             fThreads.push(SkNEW_ARGS(SkThread, (&ThreadPool::Loop, this)));
             fThreads.top()->start();
         }
     }
 
     ~ThreadPool() {
         SkASSERT(fWork.isEmpty());  // All SkTaskGroups should be destroyed by now.
-        {
-            AutoLock lock(&fReady);
-            fDraining = true;
-            fReady.broadcast();
+
+        // Send a poison pill to each thread.
+        SkAtomic<int> dummy(0);
+        for (int i = 0; i < fThreads.count(); i++) {
+            this->add(NULL, NULL, &dummy);
         }
+        // Wait for them all to swallow the pill and die.
         for (int i = 0; i < fThreads.count(); i++) {
             fThreads[i]->join();
         }
         SkASSERT(fWork.isEmpty());  // Can't hurt to double check.
         fThreads.deleteAll();
     }
 
     void add(void (*fn)(void*), void* arg, SkAtomic<int32_t>* pending) {
         Work work = { fn, arg, pending };
         pending->fetch_add(+1, sk_memory_order_relaxed);  // No barrier needed.
         {
-            AutoLock lock(&fReady);
+            AutoLock lock(&fWorkLock);
             fWork.push(work);
-            fReady.signal();
         }
+        fWorkAvailable.signal(1);
     }
 
     void batch(void (*fn)(void*), void* arg, int N, size_t stride, SkAtomic<int32_t>* pending) {
         pending->fetch_add(+N, sk_memory_order_relaxed);  // No barrier needed.
         {
-            AutoLock lock(&fReady);
+            AutoLock lock(&fWorkLock);
             Work* batch = fWork.append(N);
             for (int i = 0; i < N; i++) {
                 Work work = { fn, (char*)arg + i*stride, pending };
                 batch[i] = work;
             }
-            fReady.broadcast();
         }
+        fWorkAvailable.signal(N);
     }
 
     static void Loop(void* arg) {
         ThreadPool* pool = (ThreadPool*)arg;
         Work work;
         while (true) {
+            // Sleep until there's work available, and claim one unit of Work as we wake.
+            pool->fWorkAvailable.wait();
             {
-                AutoLock lock(&pool->fReady);
-                while (pool->fWork.isEmpty()) {
-                    if (pool->fDraining) {
-                        return;
-                    }
-                    pool->fReady.wait();
+                AutoLock lock(&pool->fWorkLock);
+                if (pool->fWork.isEmpty()) {
+                    // Someone in Wait() stole our work (fWorkAvailable is an upper bound).
+                    // Well, that's fine, back to sleep for us.
+                    continue;
                 }
                 pool->fWork.pop(&work);
             }
+            if (!work.fn) {
+                return;  // Poison pill.  Time... to die.
+            }
             work.fn(work.arg);
             work.pending->fetch_add(-1, sk_memory_order_release);  // Pairs with load in Wait().
         }
     }
 
-    SkTDArray<Work>      fWork;
+    // fWorkLock must be held when reading or modifying fWork.
+    SkSpinlock      fWorkLock;
+    SkTDArray<Work> fWork;
+
+    // A thread-safe upper bound for fWork.count().
+    //
+    // We'd have it be an exact count but for the loop in Wait():
+    // we never want that to block, so it can't call fWorkAvailable.wait(),
+    // and that's the only way to decrement fWorkAvailable.
+    // So fWorkAvailable may overcount actual the work available.
+    // We make do, but this means some worker threads may wake spuriously.
+    SkSemaphore fWorkAvailable;
+
+    // These are only changed in a single-threaded context.
     SkTDArray<SkThread*> fThreads;
-    SkCondVar            fReady;
-    bool                 fDraining;
+    static ThreadPool* gGlobal;
 
-    static ThreadPool* gGlobal;
     friend struct SkTaskGroup::Enabler;
 };
 ThreadPool* ThreadPool::gGlobal = NULL;
 
 }  // namespace
 
 SkTaskGroup::Enabler::Enabler(int threads) {
     SkASSERT(ThreadPool::gGlobal == NULL);
-    if (threads != 0 && SkCondVar::Supported()) {
+    if (threads != 0) {
         ThreadPool::gGlobal = SkNEW_ARGS(ThreadPool, (threads));
     }
 }
 
 SkTaskGroup::Enabler::~Enabler() {
     SkDELETE(ThreadPool::gGlobal);
 }
 
 SkTaskGroup::SkTaskGroup() : fPending(0) {}
 
 void SkTaskGroup::wait()                            { ThreadPool::Wait(&fPending); }
 void SkTaskGroup::add(SkRunnable* task)             { ThreadPool::Add(task, &fPending); }
 void SkTaskGroup::add(void (*fn)(void*), void* arg) { ThreadPool::Add(fn, arg, &fPending); }
 void SkTaskGroup::batch (void (*fn)(void*), void* args, int N, size_t stride) {
     ThreadPool::Batch(fn, args, N, stride, &fPending);
 }