v4.19.13 snapshot.
diff --git a/include/linux/rcu_sync.h b/include/linux/rcu_sync.h
new file mode 100644
index 0000000..ece7ed9
--- /dev/null
+++ b/include/linux/rcu_sync.h
@@ -0,0 +1,87 @@
+/*
+ * RCU-based infrastructure for lightweight reader-writer locking
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, you can access it online at
+ * http://www.gnu.org/licenses/gpl-2.0.html.
+ *
+ * Copyright (c) 2015, Red Hat, Inc.
+ *
+ * Author: Oleg Nesterov <oleg@redhat.com>
+ */
+
+#ifndef _LINUX_RCU_SYNC_H_
+#define _LINUX_RCU_SYNC_H_
+
+#include <linux/wait.h>
+#include <linux/rcupdate.h>
+
+enum rcu_sync_type { RCU_SYNC, RCU_SCHED_SYNC, RCU_BH_SYNC };
+
+/* Structure to mediate between updaters and fastpath-using readers. */
+struct rcu_sync {
+ int gp_state;
+ int gp_count;
+ wait_queue_head_t gp_wait;
+
+ int cb_state;
+ struct rcu_head cb_head;
+
+ enum rcu_sync_type gp_type;
+};
+
+extern void rcu_sync_lockdep_assert(struct rcu_sync *);
+
+/**
+ * rcu_sync_is_idle() - Are readers permitted to use their fastpaths?
+ * @rsp: Pointer to rcu_sync structure to use for synchronization
+ *
+ * Returns true if readers are permitted to use their fastpaths.
+ * Must be invoked within an RCU read-side critical section whose
+ * flavor matches that of the rcu_sync struture.
+ */
+static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
+{
+#ifdef CONFIG_PROVE_RCU
+ rcu_sync_lockdep_assert(rsp);
+#endif
+ return !rsp->gp_state; /* GP_IDLE */
+}
+
+extern void rcu_sync_init(struct rcu_sync *, enum rcu_sync_type);
+extern void rcu_sync_enter_start(struct rcu_sync *);
+extern void rcu_sync_enter(struct rcu_sync *);
+extern void rcu_sync_exit(struct rcu_sync *);
+extern void rcu_sync_dtor(struct rcu_sync *);
+
+#define __RCU_SYNC_INITIALIZER(name, type) { \
+ .gp_state = 0, \
+ .gp_count = 0, \
+ .gp_wait = __WAIT_QUEUE_HEAD_INITIALIZER(name.gp_wait), \
+ .cb_state = 0, \
+ .gp_type = type, \
+ }
+
+#define __DEFINE_RCU_SYNC(name, type) \
+ struct rcu_sync_struct name = __RCU_SYNC_INITIALIZER(name, type)
+
+#define DEFINE_RCU_SYNC(name) \
+ __DEFINE_RCU_SYNC(name, RCU_SYNC)
+
+#define DEFINE_RCU_SCHED_SYNC(name) \
+ __DEFINE_RCU_SYNC(name, RCU_SCHED_SYNC)
+
+#define DEFINE_RCU_BH_SYNC(name) \
+ __DEFINE_RCU_SYNC(name, RCU_BH_SYNC)
+
+#endif /* _LINUX_RCU_SYNC_H_ */