sched/sem_rw.c: Add downgrade_write API for sem_rw

support downgrad a write lock to a read lock

Signed-off-by: Yongrong Wang <wangyongrong@xiaomi.com>

include/nuttx/rwsem.h

@@ -142,6 +142,19 @@ void down_write(FAR rw_semaphore_t *rwsem);
 
 void up_write(FAR rw_semaphore_t *rwsem);
 
+/****************************************************************************
+ * Name: downgrade_write
+ *
+ * Description:
+ *   Down grade write lock to read lock on a read-write-lock object.
+ *
+ * Input Parameters:
+ *   rwsem  - Pointer to the read-write-lock descriptor.
+ *
+ ****************************************************************************/
+
+void downgrade_write(FAR rw_semaphore_t *rwsem);
+
 /****************************************************************************
  * Name: init_rwsem
  *

sched/semaphore/sem_rw.c

@@ -286,6 +286,33 @@ void up_write(FAR rw_semaphore_t *rwsem)
   nxmutex_unlock(&rwsem->protected);
 }
 
+/****************************************************************************
+ * Name: downgrade_write
+ *
+ * Description:
+ *   Downgrade write lock to read lock on a read-write-lock object.
+ *
+ * Input Parameters:
+ *   rwsem  - Pointer to the read-write-lock descriptor.
+ *
+ ****************************************************************************/
+
+void downgrade_write(FAR rw_semaphore_t *rwsem)
+{
+  nxmutex_lock(&rwsem->protected);
+
+  DEBUGASSERT(rwsem->writer == 1);
+  DEBUGASSERT(rwsem->reader == 0);
+  DEBUGASSERT(rwsem->holder == _SCHED_GETTID());
+
+  rwsem->writer = 0;
+  rwsem->reader++;
+  rwsem->holder = RWSEM_NO_HOLDER;
+
+  up_wait(rwsem);
+  nxmutex_unlock(&rwsem->protected);
+}
+
 /****************************************************************************
  * Name: init_rwsem
  *