--- /dev/null
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _LINUX_SIX_H
+#define _LINUX_SIX_H
+
+/**
+ * DOC: SIX locks overview
+ *
+ * Shared/intent/exclusive locks: sleepable read/write locks, like rw semaphores
+ * but with an additional state: read/shared, intent, exclusive/write
+ *
+ * The purpose of the intent state is to allow for greater concurrency on tree
+ * structures without deadlocking. In general, a read can't be upgraded to a
+ * write lock without deadlocking, so an operation that updates multiple nodes
+ * will have to take write locks for the full duration of the operation.
+ *
+ * But by adding an intent state, which is exclusive with other intent locks but
+ * not with readers, we can take intent locks at the start of the operation,
+ * and then take write locks only for the actual update to each individual
+ * nodes, without deadlocking.
+ *
+ * Example usage:
+ * six_lock_read(&foo->lock);
+ * six_unlock_read(&foo->lock);
+ *
+ * An intent lock must be held before taking a write lock:
+ * six_lock_intent(&foo->lock);
+ * six_lock_write(&foo->lock);
+ * six_unlock_write(&foo->lock);
+ * six_unlock_intent(&foo->lock);
+ *
+ * Other operations:
+ * six_trylock_read()
+ * six_trylock_intent()
+ * six_trylock_write()
+ *
+ * six_lock_downgrade() convert from intent to read
+ * six_lock_tryupgrade() attempt to convert from read to intent, may fail
+ *
+ * There are also interfaces that take the lock type as an enum:
+ *
+ * six_lock_type(&foo->lock, SIX_LOCK_read);
+ * six_trylock_convert(&foo->lock, SIX_LOCK_read, SIX_LOCK_intent)
+ * six_lock_type(&foo->lock, SIX_LOCK_write);
+ * six_unlock_type(&foo->lock, SIX_LOCK_write);
+ * six_unlock_type(&foo->lock, SIX_LOCK_intent);
+ *
+ * Lock sequence numbers - unlock(), relock():
+ *
+ * Locks embed sequences numbers, which are incremented on write lock/unlock.
+ * This allows locks to be dropped and the retaken iff the state they protect
+ * hasn't changed; this makes it much easier to avoid holding locks while e.g.
+ * doing IO or allocating memory.
+ *
+ * Example usage:
+ * six_lock_read(&foo->lock);
+ * u32 seq = six_lock_seq(&foo->lock);
+ * six_unlock_read(&foo->lock);
+ *
+ * some_operation_that_may_block();
+ *
+ * if (six_relock_read(&foo->lock, seq)) { ... }
+ *
+ * If the relock operation succeeds, it is as if the lock was never unlocked.
+ *
+ * Reentrancy:
+ *
+ * Six locks are not by themselves reentrant, but have counters for both the
+ * read and intent states that can be used to provide reentrancy by an upper
+ * layer that tracks held locks. If a lock is known to already be held in the
+ * read or intent state, six_lock_increment() can be used to bump the "lock
+ * held in this state" counter, increasing the number of unlock calls that
+ * will be required to fully unlock it.
+ *
+ * Example usage:
+ * six_lock_read(&foo->lock);
+ * six_lock_increment(&foo->lock, SIX_LOCK_read);
+ * six_unlock_read(&foo->lock);
+ * six_unlock_read(&foo->lock);
+ * foo->lock is now fully unlocked.
+ *
+ * Since the intent state supercedes read, it's legal to increment the read
+ * counter when holding an intent lock, but not the reverse.
+ *
+ * A lock may only be held once for write: six_lock_increment(.., SIX_LOCK_write)
+ * is not legal.
+ *
+ * should_sleep_fn:
+ *
+ * There is a six_lock() variant that takes a function pointer that is called
+ * immediately prior to schedule() when blocking, and may return an error to
+ * abort.
+ *
+ * One possible use for this feature is when objects being locked are part of
+ * a cache and may reused, and lock ordering is based on a property of the
+ * object that will change when the object is reused - i.e. logical key order.
+ *
+ * If looking up an object in the cache may race with object reuse, and lock
+ * ordering is required to prevent deadlock, object reuse may change the
+ * correct lock order for that object and cause a deadlock. should_sleep_fn
+ * can be used to check if the object is still the object we want and avoid
+ * this deadlock.
+ *
+ * Wait list entry interface:
+ *
+ * There is a six_lock() variant, six_lock_waiter(), that takes a pointer to a
+ * wait list entry. By embedding six_lock_waiter into another object, and by
+ * traversing lock waitlists, it is then possible for an upper layer to
+ * implement full cycle detection for deadlock avoidance.
+ *
+ * should_sleep_fn should be used for invoking the cycle detector, walking the
+ * graph of held locks to check for a deadlock. The upper layer must track
+ * held locks for each thread, and each thread's held locks must be reachable
+ * from its six_lock_waiter object.
+ *
+ * six_lock_waiter() will add the wait object to the waitlist re-trying taking
+ * the lock, and before calling should_sleep_fn, and the wait object will not
+ * be removed from the waitlist until either the lock has been successfully
+ * acquired, or we aborted because should_sleep_fn returned an error.
+ *
+ * Also, six_lock_waiter contains a timestamp, and waiters on a waitlist will
+ * have timestamps in strictly ascending order - this is so the timestamp can
+ * be used as a cursor for lock graph traverse.
+ */
+
+#include <linux/lockdep.h>
+#include <linux/sched.h>
+#include <linux/types.h>
+
+enum six_lock_type {
+ SIX_LOCK_read,
+ SIX_LOCK_intent,
+ SIX_LOCK_write,
+};
+
+struct six_lock {
+ atomic_t state;
+ u32 seq;
+ unsigned intent_lock_recurse;
+ struct task_struct *owner;
+ unsigned __percpu *readers;
+ raw_spinlock_t wait_lock;
+ struct list_head wait_list;
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ struct lockdep_map dep_map;
+#endif
+};
+
+struct six_lock_waiter {
+ struct list_head list;
+ struct task_struct *task;
+ enum six_lock_type lock_want;
+ bool lock_acquired;
+ u64 start_time;
+};
+
+typedef int (*six_lock_should_sleep_fn)(struct six_lock *lock, void *);
+
+void six_lock_exit(struct six_lock *lock);
+
+enum six_lock_init_flags {
+ SIX_LOCK_INIT_PCPU = 1U << 0,
+};
+
+void __six_lock_init(struct six_lock *lock, const char *name,
+ struct lock_class_key *key, enum six_lock_init_flags flags);
+
+/**
+ * six_lock_init - initialize a six lock
+ * @lock: lock to initialize
+ * @flags: optional flags, i.e. SIX_LOCK_INIT_PCPU
+ */
+#define six_lock_init(lock, flags) \
+do { \
+ static struct lock_class_key __key; \
+ \
+ __six_lock_init((lock), #lock, &__key, flags); \
+} while (0)
+
+/**
+ * six_lock_seq - obtain current lock sequence number
+ * @lock: six_lock to obtain sequence number for
+ *
+ * @lock should be held for read or intent, and not write
+ *
+ * By saving the lock sequence number, we can unlock @lock and then (typically
+ * after some blocking operation) attempt to relock it: the relock will succeed
+ * if the sequence number hasn't changed, meaning no write locks have been taken
+ * and state corresponding to what @lock protects is still valid.
+ */
+static inline u32 six_lock_seq(const struct six_lock *lock)
+{
+ return lock->seq;
+}
+
+bool six_trylock_ip(struct six_lock *lock, enum six_lock_type type, unsigned long ip);
+
+/**
+ * six_trylock_type - attempt to take a six lock without blocking
+ * @lock: lock to take
+ * @type: SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
+ *
+ * Return: true on success, false on failure.
+ */
+static inline bool six_trylock_type(struct six_lock *lock, enum six_lock_type type)
+{
+ return six_trylock_ip(lock, type, _THIS_IP_);
+}
+
+int six_lock_ip_waiter(struct six_lock *lock, enum six_lock_type type,
+ struct six_lock_waiter *wait,
+ six_lock_should_sleep_fn should_sleep_fn, void *p,
+ unsigned long ip);
+
+/**
+ * six_lock_waiter - take a lock, with full waitlist interface
+ * @lock: lock to take
+ * @type: SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
+ * @wait: pointer to wait object, which will be added to lock's waitlist
+ * @should_sleep_fn: callback run after adding to waitlist, immediately prior
+ * to scheduling
+ * @p: passed through to @should_sleep_fn
+ *
+ * This is a convenience wrapper around six_lock_ip_waiter(), see that function
+ * for full documentation.
+ *
+ * Return: 0 on success, or the return code from @should_sleep_fn on failure.
+ */
+static inline int six_lock_waiter(struct six_lock *lock, enum six_lock_type type,
+ struct six_lock_waiter *wait,
+ six_lock_should_sleep_fn should_sleep_fn, void *p)
+{
+ return six_lock_ip_waiter(lock, type, wait, should_sleep_fn, p, _THIS_IP_);
+}
+
+/**
+ * six_lock_ip - take a six lock lock
+ * @lock: lock to take
+ * @type: SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
+ * @should_sleep_fn: callback run after adding to waitlist, immediately prior
+ * to scheduling
+ * @p: passed through to @should_sleep_fn
+ * @ip: ip parameter for lockdep/lockstat, i.e. _THIS_IP_
+ *
+ * Return: 0 on success, or the return code from @should_sleep_fn on failure.
+ */
+static inline int six_lock_ip(struct six_lock *lock, enum six_lock_type type,
+ six_lock_should_sleep_fn should_sleep_fn, void *p,
+ unsigned long ip)
+{
+ struct six_lock_waiter wait;
+
+ return six_lock_ip_waiter(lock, type, &wait, should_sleep_fn, p, ip);
+}
+
+/**
+ * six_lock_type - take a six lock lock
+ * @lock: lock to take
+ * @type: SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
+ * @should_sleep_fn: callback run after adding to waitlist, immediately prior
+ * to scheduling
+ * @p: passed through to @should_sleep_fn
+ *
+ * Return: 0 on success, or the return code from @should_sleep_fn on failure.
+ */
+static inline int six_lock_type(struct six_lock *lock, enum six_lock_type type,
+ six_lock_should_sleep_fn should_sleep_fn, void *p)
+{
+ struct six_lock_waiter wait;
+
+ return six_lock_ip_waiter(lock, type, &wait, should_sleep_fn, p, _THIS_IP_);
+}
+
+bool six_relock_ip(struct six_lock *lock, enum six_lock_type type,
+ unsigned seq, unsigned long ip);
+
+/**
+ * six_relock_type - attempt to re-take a lock that was held previously
+ * @lock: lock to take
+ * @type: SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
+ * @seq: lock sequence number obtained from six_lock_seq() while lock was
+ * held previously
+ *
+ * Return: true on success, false on failure.
+ */
+static inline bool six_relock_type(struct six_lock *lock, enum six_lock_type type,
+ unsigned seq)
+{
+ return six_relock_ip(lock, type, seq, _THIS_IP_);
+}
+
+void six_unlock_ip(struct six_lock *lock, enum six_lock_type type, unsigned long ip);
+
+/**
+ * six_unlock_type - drop a six lock
+ * @lock: lock to unlock
+ * @type: SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
+ *
+ * When a lock is held multiple times (because six_lock_incement()) was used),
+ * this decrements the 'lock held' counter by one.
+ *
+ * For example:
+ * six_lock_read(&foo->lock); read count 1
+ * six_lock_increment(&foo->lock, SIX_LOCK_read); read count 2
+ * six_lock_unlock(&foo->lock, SIX_LOCK_read); read count 1
+ * six_lock_unlock(&foo->lock, SIX_LOCK_read); read count 0
+ */
+static inline void six_unlock_type(struct six_lock *lock, enum six_lock_type type)
+{
+ six_unlock_ip(lock, type, _THIS_IP_);
+}
+
+#define __SIX_LOCK(type) \
+static inline bool six_trylock_ip_##type(struct six_lock *lock, unsigned long ip)\
+{ \
+ return six_trylock_ip(lock, SIX_LOCK_##type, ip); \
+} \
+ \
+static inline bool six_trylock_##type(struct six_lock *lock) \
+{ \
+ return six_trylock_ip(lock, SIX_LOCK_##type, _THIS_IP_); \
+} \
+ \
+static inline int six_lock_ip_waiter_##type(struct six_lock *lock, \
+ struct six_lock_waiter *wait, \
+ six_lock_should_sleep_fn should_sleep_fn, void *p,\
+ unsigned long ip) \
+{ \
+ return six_lock_ip_waiter(lock, SIX_LOCK_##type, wait, should_sleep_fn, p, ip);\
+} \
+ \
+static inline int six_lock_ip_##type(struct six_lock *lock, \
+ six_lock_should_sleep_fn should_sleep_fn, void *p, \
+ unsigned long ip) \
+{ \
+ return six_lock_ip(lock, SIX_LOCK_##type, should_sleep_fn, p, ip);\
+} \
+ \
+static inline bool six_relock_ip_##type(struct six_lock *lock, u32 seq, unsigned long ip)\
+{ \
+ return six_relock_ip(lock, SIX_LOCK_##type, seq, ip); \
+} \
+ \
+static inline bool six_relock_##type(struct six_lock *lock, u32 seq) \
+{ \
+ return six_relock_ip(lock, SIX_LOCK_##type, seq, _THIS_IP_); \
+} \
+ \
+static inline int six_lock_##type(struct six_lock *lock, \
+ six_lock_should_sleep_fn fn, void *p)\
+{ \
+ return six_lock_ip_##type(lock, fn, p, _THIS_IP_); \
+} \
+ \
+static inline void six_unlock_ip_##type(struct six_lock *lock, unsigned long ip) \
+{ \
+ six_unlock_ip(lock, SIX_LOCK_##type, ip); \
+} \
+ \
+static inline void six_unlock_##type(struct six_lock *lock) \
+{ \
+ six_unlock_ip(lock, SIX_LOCK_##type, _THIS_IP_); \
+}
+
+__SIX_LOCK(read)
+__SIX_LOCK(intent)
+__SIX_LOCK(write)
+#undef __SIX_LOCK
+
+void six_lock_downgrade(struct six_lock *);
+bool six_lock_tryupgrade(struct six_lock *);
+bool six_trylock_convert(struct six_lock *, enum six_lock_type,
+ enum six_lock_type);
+
+void six_lock_increment(struct six_lock *, enum six_lock_type);
+
+void six_lock_wakeup_all(struct six_lock *);
+
+struct six_lock_count {
+ unsigned n[3];
+};
+
+struct six_lock_count six_lock_counts(struct six_lock *);
+void six_lock_readers_add(struct six_lock *, int);
+
+#endif /* _LINUX_SIX_H */
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