support SharedMutex.

src/bthread/shared_mutex.cpp

@@ -0,0 +1,47 @@
+#include "shared_mutex.h"
+namespace bthread {
+SharedMutex::SharedMutex(): _reader_count(0), _reader_wait(0) {
+    _writer_butex = butex_create();
+    _reader_butex = butex_create();
+}
+
+void SharedMutex::lock_shared() {
+    if (_reader_count.fetch_add(1) < 0) {
+        butex_wait(_reader_butex, _reader_butex, nullptr);
+    }
+}
+
+void SharedMutex::unlock_shared() {
+    int32_t r = _reader_count.fetch_add(-1);
+    if (r < 0) {
+        runlock_slow(r);
+    }
+}
+
+void SharedMutex::unlock_shared_slow(r int32) {
+    if (r == 0 || r == -max_readers) {
+        throw std::system_error(std::errc::operation_not_permitted, "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);
+    }
+}
+
+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 +1,30 @@
+#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(r int32);
+
+    static constexpr int32_t max_readers = 1 << 30;
+    Mutex _w;
+    void* _writer_butex;
+    void* _reader_butex;
+    butil::atomic<int32_t> _reader_count;
+    butil::atomic<int32_t> _reader_wait;
+};
+}
+
+#endif  //BTHREAD_SHARED_MUTEX_H