percpu-rwsem: introduce percpu_down_read_trylock()

Add percpu_down_read_trylock(), it will have the user soon.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>

include/linux/percpu-rwsem.h

@@ -16,6 +16,7 @@ struct percpu_rw_semaphore {
 };
 
 extern void percpu_down_read(struct percpu_rw_semaphore *);
+extern int  percpu_down_read_trylock(struct percpu_rw_semaphore *);
 extern void percpu_up_read(struct percpu_rw_semaphore *);
 
 extern void percpu_down_write(struct percpu_rw_semaphore *);

kernel/locking/percpu-rwsem.c

@@ -88,6 +88,19 @@ void percpu_down_read(struct percpu_rw_semaphore *brw)
 	__up_read(&brw->rw_sem);
 }
 
+int percpu_down_read_trylock(struct percpu_rw_semaphore *brw)
+{
+	if (unlikely(!update_fast_ctr(brw, +1))) {
+		if (!__down_read_trylock(&brw->rw_sem))
+			return 0;
+		atomic_inc(&brw->slow_read_ctr);
+		__up_read(&brw->rw_sem);
+	}
+
+	rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_);
+	return 1;
+}
+
 void percpu_up_read(struct percpu_rw_semaphore *brw)
 {
 	rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);