Fix swapping of read/write mutex in implementation of SharedMutex.

include/mutex.hpp

@@ -7,8 +7,11 @@ namespace rack {
 
 
 /** Allows multiple "reader" threads to obtain a lock simultaneously, but only one "writer" thread.
+
 This implementation is currently just a wrapper for pthreads, which works on Linux/Mac/.
 This is available in C++17 as std::shared_mutex, but unfortunately we're using C++11.
+
+Locking should be avoided in real-time audio threads.
 */
 struct SharedMutex {
 	pthread_rwlock_t rwlock;
@@ -22,12 +25,12 @@ struct SharedMutex {
 	}
 
 	void lock() {
-		if (pthread_rwlock_rdlock(&rwlock))
-			throw Exception("pthread_rwlock_rdlock failed");
+		if (pthread_rwlock_wrlock(&rwlock))
+			throw Exception("pthread_rwlock_wrlock failed");
 	}
 	/** Returns whether the lock was acquired. */
 	bool try_lock() {
-		return pthread_rwlock_tryrdlock(&rwlock) == 0;
+		return pthread_rwlock_trywrlock(&rwlock) == 0;
 	}
 	void unlock() {
 		if (pthread_rwlock_unlock(&rwlock))
@@ -35,16 +38,15 @@ struct SharedMutex {
 	}
 
 	void lock_shared() {
-		if (pthread_rwlock_wrlock(&rwlock))
-			throw Exception("pthread_rwlock_wrlock failed");
+		if (pthread_rwlock_rdlock(&rwlock))
+			throw Exception("pthread_rwlock_rdlock failed");
 	}
 	/** Returns whether the lock was acquired. */
 	bool try_lock_shared() {
-		return pthread_rwlock_trywrlock(&rwlock) == 0;
+		return pthread_rwlock_tryrdlock(&rwlock) == 0;
 	}
 	void unlock_shared() {
-		if (pthread_rwlock_unlock(&rwlock))
-			throw Exception("pthread_rwlock_unlock failed");
+		unlock();
 	}
 };