--- /dev/null
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkSharedMutex.h"
+
+#include "SkAtomics.h"
+#include "SkSemaphore.h"
+#include "SkTypes.h"
+
+// The fQueueCounts fields holds many counts in an int32_t in order to make managing them atomic.
+// These three counts must be the same size, so each gets 10 bits. The 10 bits represent
+// the log of the count which is 1024.
+//
+// The three counts held in fQueueCounts are:
+// * Shared - the number of shared lock holders currently running.
+// * WaitingExclusive - the number of threads waiting for an exclusive lock.
+// * WaitingShared - the number of threads waiting to run while waiting for an exclusive thread
+// to finish.
+static const int kLogThreadCount = 10;
+
+enum {
+ kSharedOffset = (0 * kLogThreadCount),
+ kWaitingExlusiveOffset = (1 * kLogThreadCount),
+ kWaitingSharedOffset = (2 * kLogThreadCount),
+ kSharedMask = ((1 << kLogThreadCount) - 1) << kSharedOffset,
+ kWaitingExclusiveMask = ((1 << kLogThreadCount) - 1) << kWaitingExlusiveOffset,
+ kWaitingSharedMask = ((1 << kLogThreadCount) - 1) << kWaitingSharedOffset,
+};
+
+SkSharedMutex::SkSharedMutex() : fQueueCounts(0) { }
+
+void SkSharedMutex::acquire() {
+ // Increment the count of exclusive queue waiters.
+ int32_t oldQueueCounts = fQueueCounts.fetch_add(1 << kWaitingExlusiveOffset,
+ sk_memory_order_acquire);
+
+ // If there are no other exclusive waiters and no shared threads are running then run
+ // else wait.
+ if ((oldQueueCounts & kWaitingExclusiveMask) > 0 || (oldQueueCounts & kSharedMask) > 0) {
+ fExclusiveQueue.wait();
+ }
+}
+
+void SkSharedMutex::release() {
+ int32_t oldQueueCounts = fQueueCounts.load(sk_memory_order_relaxed);
+ int32_t waitingShared;
+ int32_t newQueueCounts;
+ do {
+ newQueueCounts = oldQueueCounts;
+
+ // Decrement exclusive waiters.
+ newQueueCounts -= 1 << kWaitingExlusiveOffset;
+
+ // The number of threads waiting to acquire a shared lock.
+ waitingShared = (oldQueueCounts & kWaitingSharedMask) >> kWaitingSharedOffset;
+
+ // If there are any move the counts of all the shared waiters to actual shared. They are
+ // going to run next.
+ if (waitingShared > 0) {
+
+ // Set waiting shared to zero.
+ newQueueCounts &= ~kWaitingSharedMask;
+
+ // Because this is the exclusive release, then there are zero readers. So, the bits
+ // for shared locks should be zero. Since those bits are zero, we can just |= in the
+ // waitingShared count instead of clearing with an &= and then |= the count.
+ newQueueCounts |= waitingShared << kSharedOffset;
+ }
+
+ } while (!fQueueCounts.compare_exchange(&oldQueueCounts, newQueueCounts,
+ sk_memory_order_release, sk_memory_order_relaxed));
+
+ if (waitingShared > 0) {
+ // Run all the shared.
+ fSharedQueue.signal(waitingShared);
+ } else if ((newQueueCounts & kWaitingExclusiveMask) > 0) {
+ // Run a single exclusive waiter.
+ fExclusiveQueue.signal();
+ }
+}
+
+void SkSharedMutex::acquireShared() {
+ int32_t oldQueueCounts = fQueueCounts.load(sk_memory_order_relaxed);
+ int32_t newQueueCounts;
+ do {
+ newQueueCounts = oldQueueCounts;
+ // If there are waiting exclusives then this shared lock waits else it runs.
+ if ((newQueueCounts & kWaitingExclusiveMask) > 0) {
+ newQueueCounts += 1 << kWaitingSharedOffset;
+ } else {
+ newQueueCounts += 1 << kSharedOffset;
+ }
+ } while (!fQueueCounts.compare_exchange(&oldQueueCounts, newQueueCounts,
+ sk_memory_order_acquire, sk_memory_order_relaxed));
+
+ // If there are waiting exclusives, then this shared waits until after it runs.
+ if ((newQueueCounts & kWaitingExclusiveMask) > 0) {
+ fSharedQueue.wait();
+ }
+}
+
+void SkSharedMutex::releaseShared() {
+ // Decrement the shared count.
+ int32_t oldQueueCounts = fQueueCounts.fetch_add(-1 << kSharedOffset,
+ sk_memory_order_release);
+
+ // If shared count is going to zero (because the old count == 1) and there are exclusive
+ // waiters, then run a single exclusive waiter.
+ if (((oldQueueCounts & kSharedMask) >> kSharedOffset) == 1
+ && (oldQueueCounts & kWaitingExclusiveMask) > 0) {
+ fExclusiveQueue.signal();
+ }
+}
--- /dev/null
+/*
+ * 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 SkSharedLock_DEFINED
+#define SkSharedLock_DEFINED
+
+#include "SkAtomics.h"
+#include "SkSemaphore.h"
+#include "SkTypes.h"
+
+// This is a shared lock implementation similar to pthreads rwlocks. This implementation is
+// cribbed from Preshing's article:
+// http://preshing.com/20150316/semaphores-are-surprisingly-versatile/
+//
+// This lock does not obey strict queue ordering. It will always alternate between readers and
+// a single writer.
+class SkSharedMutex {
+public:
+ SkSharedMutex();
+
+ // Acquire lock for exclusive use.
+ void acquire();
+
+ // Release lock for exclusive use.
+ void release();
+
+ // Acquire lock for shared use.
+ void acquireShared();
+
+ // Release lock for shared use.
+ void releaseShared();
+
+private:
+ SkAtomic<int32_t> fQueueCounts;
+ SkSemaphore fSharedQueue;
+ SkSemaphore fExclusiveQueue;
+};
+
+#endif // SkSharedLock_DEFINED
--- /dev/null
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkSharedMutex.h"
+#include "SkTaskGroup.h"
+
+#include "Test.h"
+
+DEF_TEST(SkSharedMutexBasic, r) {
+ SkSharedMutex sm;
+ sm.acquire();
+ sm.release();
+ sm.acquireShared();
+ sm.releaseShared();
+}
+
+DEF_TEST(SkSharedMutexMultiThreaded, r) {
+ SkSharedMutex sm;
+ static const int kSharedSize = 10;
+ int shared[kSharedSize];
+ int value = 0;
+ for (int i = 0; i < kSharedSize; ++i) {
+ shared[i] = 0;
+ }
+ sk_parallel_for(8, [&](int threadIndex) {
+ if (threadIndex % 4 != 0) {
+ for (int c = 0; c < 100000; ++c) {
+ sm.acquireShared();
+ int v = shared[0];
+ for (int i = 1; i < kSharedSize; ++i) {
+ REPORTER_ASSERT(r, v == shared[i]);
+ }
+ sm.releaseShared();
+ }
+ } else {
+ for (int c = 0; c < 100000; ++c) {
+ sm.acquire();
+ value += 1;
+ for (int i = 0; i < kSharedSize; ++i) {
+ shared[i] = value;
+ }
+ sm.release();
+ }
+ }
+ });
+}