srcu: Add might_sleep() annotation to synchronize_srcu()

Although synchronize_srcu() can sleep, it will not sleep if the fast
path succeeds, which means that illegal use of synchronize_rcu()
might go unnoticed.  This commit therefore adds might_sleep(), which
unconditionally catches illegal use of synchronize_rcu() from atomic
context.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/kernel/srcu.c b/kernel/srcu.c
index c9d003b..3e43a21 100644
--- a/kernel/srcu.c
+++ b/kernel/srcu.c
@@ -421,6 +421,7 @@
 			   !lock_is_held(&rcu_sched_lock_map),
 			   "Illegal synchronize_srcu() in same-type SRCU (or RCU) read-side critical section");
 
+	might_sleep();
 	init_completion(&rcu.completion);
 
 	head->next = NULL;