rw_locking: add new primitives for lock acquiring/releasing required when switching from read to write access

Using the lock_start_read/lock_stop_read primitives would cause a deadlock if two or more readers would switch to writing

Author: rvlad-patrascu

rw_locking.h

@@ -30,6 +30,7 @@
 typedef struct rw_lock_t {
 	gen_lock_t *lock;
 	int w_flag;
+	int sw_flag;
 	int r_count;
 } rw_lock_t;
 
@@ -113,7 +114,30 @@ inline static void lock_destroy_rw(rw_lock_t *_lock)
 		lock_release((_lock)->lock); \
 	} while (0)
 
-/* switch to writing access with lock previously acquired for reading
+#define lock_start_sw_read(_lock) \
+	do { \
+		__label__ again; \
+		again: \
+			lock_get((_lock)->lock); \
+			if ((_lock)->w_flag || (_lock)->sw_flag) { \
+				lock_release((_lock)->lock); \
+				usleep(LOCK_WAIT); \
+				goto again; \
+			} \
+			(_lock)->r_count++; \
+			(_lock)->sw_flag = 1; \
+			lock_release((_lock)->lock); \
+	} while (0)
+
+#define lock_stop_sw_read(_lock) \
+	do { \
+		lock_get((_lock)->lock); \
+		(_lock)->r_count--; \
+		lock_release((_lock)->lock); \
+		(_lock)->sw_flag = 0; \
+	} while (0)
+
+/* switch to writing access with lock previously acquired for switchable reading
  * note: switching back to reading is required before releasing the lock
  */
 #define lock_switch_write(_lock, __old) \