support SharedMutex.

src/bthread/shared_mutex.cpp

@@ -0,0 +1,71 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include "shared_mutex.h"
+#include "butex.h"
+
+namespace bthread {
+SharedMutex::SharedMutex(): _reader_count(0), _reader_wait(0) {
+    _writer_butex = butex_create_checked<uint32_t>();
+    *_writer_butex = 0;
+    _reader_butex = butex_create_checked<uint32_t>();
+    *_reader_butex = 0;
+}
+
+void SharedMutex::lock_shared() {
+    if (_reader_count.fetch_add(1) < 0) {
+        butex_wait(_reader_butex, 0, nullptr);
+    }
+}
+
+void SharedMutex::unlock_shared() {
+    int32_t r = _reader_count.fetch_add(-1);
+    if (r < 0) {
+        unlock_shared_slow(r);
+    }
+}
+
+void SharedMutex::unlock_shared_slow(int32_t r) {
+    if (r == 0 || r == -max_readers) {
+        throw std::system_error(std::error_code(static_cast<int>(std::errc::operation_not_permitted),
+                                                std::system_category()),
+                                "unlock of unlocked SharedMutex");
+    }
+    if (_reader_wait.fetch_add(-1) == 1) {
+        butex_wake(_writer_butex);
+    }
+}
+
+void SharedMutex::lock() {
+    _w.lock();
+    int32_t r = _reader_count.fetch_add(-max_readers);
+    if (r != 0 && _reader_wait.fetch_add(r) + r != 0) {
+        butex_wait(_writer_butex, 0, nullptr);
+    }
+}
+
+void SharedMutex::unlock() {
+    int32_t r = _reader_count.fetch_add(max_readers);
+    for(int32_t i = 0; i < r; i++) {
+        butex_wake(_reader_butex);
+    } 
+    _w.unlock();
+}
+}
+
+
+

src/bthread/shared_mutex.h

@@ -0,0 +1,47 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#ifndef  BTHREAD_SHARED_MUTEX_H
+#define  BTHREAD_SHARED_MUTEX_H
+#include "mutex.h"
+
+namespace bthread {
+
+// compatible with c++17 std::shared_mutex, migration from golang
+// see https://github.com/golang/go/blob/master/src/sync/rwmutex.go
+class SharedMutex {
+public:
+    SharedMutex();
+    void lock_shared();
+    void unlock_shared();
+    void lock();
+    void unlock();
+
+private:
+    DISALLOW_COPY_AND_ASSIGN(SharedMutex);
+    void unlock_shared_slow(int32_t r);
+
+    static constexpr int32_t max_readers = 1 << 30;
+    Mutex _w;
+    uint32_t* _writer_butex;
+    uint32_t* _reader_butex;
+    butil::atomic<int32_t> _reader_count;
+    butil::atomic<int32_t> _reader_wait;
+};
+}
+
+#endif  //BTHREAD_SHARED_MUTEX_H