v4.19.13 snapshot.
diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
new file mode 100644
index 0000000..ab93b6e
--- /dev/null
+++ b/include/linux/rwsem.h
@@ -0,0 +1,196 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* rwsem.h: R/W semaphores, public interface
+ *
+ * Written by David Howells (dhowells@redhat.com).
+ * Derived from asm-i386/semaphore.h
+ */
+
+#ifndef _LINUX_RWSEM_H
+#define _LINUX_RWSEM_H
+
+#include <linux/linkage.h>
+
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/list.h>
+#include <linux/spinlock.h>
+#include <linux/atomic.h>
+#include <linux/err.h>
+#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
+#include <linux/osq_lock.h>
+#endif
+
+struct rw_semaphore;
+
+#ifdef CONFIG_RWSEM_GENERIC_SPINLOCK
+#include <linux/rwsem-spinlock.h> /* use a generic implementation */
+#define __RWSEM_INIT_COUNT(name) .count = RWSEM_UNLOCKED_VALUE
+#else
+/* All arch specific implementations share the same struct */
+struct rw_semaphore {
+ atomic_long_t count;
+ struct list_head wait_list;
+ raw_spinlock_t wait_lock;
+#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
+ struct optimistic_spin_queue osq; /* spinner MCS lock */
+ /*
+ * Write owner. Used as a speculative check to see
+ * if the owner is running on the cpu.
+ */
+ struct task_struct *owner;
+#endif
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ struct lockdep_map dep_map;
+#endif
+};
+
+/*
+ * Setting bit 0 of the owner field with other non-zero bits will indicate
+ * that the rwsem is writer-owned with an unknown owner.
+ */
+#define RWSEM_OWNER_UNKNOWN ((struct task_struct *)-1L)
+
+extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem);
+extern struct rw_semaphore *rwsem_down_read_failed_killable(struct rw_semaphore *sem);
+extern struct rw_semaphore *rwsem_down_write_failed(struct rw_semaphore *sem);
+extern struct rw_semaphore *rwsem_down_write_failed_killable(struct rw_semaphore *sem);
+extern struct rw_semaphore *rwsem_wake(struct rw_semaphore *);
+extern struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem);
+
+/* Include the arch specific part */
+#include <asm/rwsem.h>
+
+/* In all implementations count != 0 means locked */
+static inline int rwsem_is_locked(struct rw_semaphore *sem)
+{
+ return atomic_long_read(&sem->count) != 0;
+}
+
+#define __RWSEM_INIT_COUNT(name) .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
+#endif
+
+/* Common initializer macros and functions */
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+# define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname }
+#else
+# define __RWSEM_DEP_MAP_INIT(lockname)
+#endif
+
+#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
+#define __RWSEM_OPT_INIT(lockname) , .osq = OSQ_LOCK_UNLOCKED, .owner = NULL
+#else
+#define __RWSEM_OPT_INIT(lockname)
+#endif
+
+#define __RWSEM_INITIALIZER(name) \
+ { __RWSEM_INIT_COUNT(name), \
+ .wait_list = LIST_HEAD_INIT((name).wait_list), \
+ .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock) \
+ __RWSEM_OPT_INIT(name) \
+ __RWSEM_DEP_MAP_INIT(name) }
+
+#define DECLARE_RWSEM(name) \
+ struct rw_semaphore name = __RWSEM_INITIALIZER(name)
+
+extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
+ struct lock_class_key *key);
+
+#define init_rwsem(sem) \
+do { \
+ static struct lock_class_key __key; \
+ \
+ __init_rwsem((sem), #sem, &__key); \
+} while (0)
+
+/*
+ * This is the same regardless of which rwsem implementation that is being used.
+ * It is just a heuristic meant to be called by somebody alreadying holding the
+ * rwsem to see if somebody from an incompatible type is wanting access to the
+ * lock.
+ */
+static inline int rwsem_is_contended(struct rw_semaphore *sem)
+{
+ return !list_empty(&sem->wait_list);
+}
+
+/*
+ * lock for reading
+ */
+extern void down_read(struct rw_semaphore *sem);
+extern int __must_check down_read_killable(struct rw_semaphore *sem);
+
+/*
+ * trylock for reading -- returns 1 if successful, 0 if contention
+ */
+extern int down_read_trylock(struct rw_semaphore *sem);
+
+/*
+ * lock for writing
+ */
+extern void down_write(struct rw_semaphore *sem);
+extern int __must_check down_write_killable(struct rw_semaphore *sem);
+
+/*
+ * trylock for writing -- returns 1 if successful, 0 if contention
+ */
+extern int down_write_trylock(struct rw_semaphore *sem);
+
+/*
+ * release a read lock
+ */
+extern void up_read(struct rw_semaphore *sem);
+
+/*
+ * release a write lock
+ */
+extern void up_write(struct rw_semaphore *sem);
+
+/*
+ * downgrade write lock to read lock
+ */
+extern void downgrade_write(struct rw_semaphore *sem);
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+/*
+ * nested locking. NOTE: rwsems are not allowed to recurse
+ * (which occurs if the same task tries to acquire the same
+ * lock instance multiple times), but multiple locks of the
+ * same lock class might be taken, if the order of the locks
+ * is always the same. This ordering rule can be expressed
+ * to lockdep via the _nested() APIs, but enumerating the
+ * subclasses that are used. (If the nesting relationship is
+ * static then another method for expressing nested locking is
+ * the explicit definition of lock class keys and the use of
+ * lockdep_set_class() at lock initialization time.
+ * See Documentation/locking/lockdep-design.txt for more details.)
+ */
+extern void down_read_nested(struct rw_semaphore *sem, int subclass);
+extern void down_write_nested(struct rw_semaphore *sem, int subclass);
+extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
+extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
+
+# define down_write_nest_lock(sem, nest_lock) \
+do { \
+ typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
+ _down_write_nest_lock(sem, &(nest_lock)->dep_map); \
+} while (0);
+
+/*
+ * Take/release a lock when not the owner will release it.
+ *
+ * [ This API should be avoided as much as possible - the
+ * proper abstraction for this case is completions. ]
+ */
+extern void down_read_non_owner(struct rw_semaphore *sem);
+extern void up_read_non_owner(struct rw_semaphore *sem);
+#else
+# define down_read_nested(sem, subclass) down_read(sem)
+# define down_write_nest_lock(sem, nest_lock) down_write(sem)
+# define down_write_nested(sem, subclass) down_write(sem)
+# define down_write_killable_nested(sem, subclass) down_write_killable(sem)
+# define down_read_non_owner(sem) down_read(sem)
+# define up_read_non_owner(sem) up_read(sem)
+#endif
+
+#endif /* _LINUX_RWSEM_H */