six locks: Documentation, renaming

 - Expanded and revamped overview documentation in six.h, giving an
   overview of all features
 - docbook-comments for all external interfaces
 - Rename some functions for simplicity, i.e.
   six_lock_ip_type() -> six_lock_ip()

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

1 files changed, 217 insertions, 81 deletions

diff --git a/fs/bcachefs/six.h b/fs/bcachefs/six.h
index 449589f76628..82bf9de72490 100644
--- a/fs/bcachefs/six.h
+++ b/fs/bcachefs/six.h
@@ -3,59 +3,124 @@
 #ifndef _LINUX_SIX_H
 #define _LINUX_SIX_H
 
-/*
- * Shared/intent/exclusive locks: sleepable read/write locks, much like rw
- * semaphores, except with a third intermediate state, intent. Basic operations
- * are:
+/**
+ * DOC: SIX locks overview
  *
- * six_lock_read(&foo->lock);
- * six_unlock_read(&foo->lock);
+ * Shared/intent/exclusive locks: sleepable read/write locks, like rw semaphores
+ * but with an additional state: read/shared, intent, exclusive/write
  *
- * six_lock_intent(&foo->lock);
- * six_unlock_intent(&foo->lock);
+ * 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.
  *
- * six_lock_write(&foo->lock);
- * six_unlock_write(&foo->lock);
+ * But by adding an intent state, which is exclusive with other intent locks but
+ * not with readers, we can take intent locks at thte start of the operation,
+ * and then take write locks only for the actual update to each individual
+ * nodes, without deadlocking.
  *
- * Intent locks block other intent locks, but do not block read locks, and you
- * must have an intent lock held before taking a write lock, like so:
+ * Example usage:
+ *   six_lock_read(&foo->lock);
+ *   six_unlock_read(&foo->lock);
  *
- * six_lock_intent(&foo->lock);
- * six_lock_write(&foo->lock);
- * six_unlock_write(&foo->lock);
- * six_unlock_intent(&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
- *
- * Locks also embed a sequence number, which is incremented when the lock is
- * locked or unlocked for write. The current sequence number can be grabbed
- * while a lock is held from lock->state.seq; then, if you drop the lock you can
- * use six_relock_(read|intent_write)(lock, seq) to attempt to retake the lock
- * iff it hasn't been locked for write in the meantime.
- *
- * There are also operations that take the lock type as a parameter, where the
- * type is one of SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write:
- *
- *   six_lock_type(lock, type)
- *   six_unlock_type(lock, type)
- *   six_relock(lock, type, seq)
- *   six_trylock_type(lock, type)
- *   six_trylock_convert(lock, from, to)
- *
- * A lock may be held multiple times by the same thread (for read or intent,
- * not write). However, the six locks code does _not_ implement the actual
- * recursive checks itself though - rather, if your code (e.g. btree iterator
- * code) knows that the current thread already has a lock held, and for the
- * correct type, six_lock_increment() may be used to bump up the counter for
- * that type - the only effect is that one more call to unlock will be required
- * before the lock is unlocked.
+ *   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 reentrent, but have counters for both the
+ *   read and intent states that can be used to provide reentrency 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>
@@ -66,8 +131,6 @@
 #include <linux/osq_lock.h>
 #endif
 
-#define SIX_LOCK_SEPARATE_LOCKFNS
-
 enum six_lock_type {
 	SIX_LOCK_read,
 	SIX_LOCK_intent,
@@ -108,6 +171,11 @@ enum six_lock_init_flags {
 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;				\
@@ -115,73 +183,148 @@ do {									\
 	__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 atomic64_read(&lock->state) >> 32;
 }
 
-bool six_trylock_ip_type(struct six_lock *lock, enum six_lock_type type,
-			 unsigned long ip);
+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_type(lock, type, _THIS_IP_);
+	return six_trylock_ip(lock, type, _THIS_IP_);
 }
 
-int six_lock_type_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);
-
-static inline int six_lock_type_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)
+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_type_ip_waiter(lock, type, wait, should_sleep_fn, p, _THIS_IP_);
+	return six_lock_ip_waiter(lock, type, wait, should_sleep_fn, p, _THIS_IP_);
 }
 
-static inline int six_lock_ip_type(struct six_lock *lock, enum six_lock_type type,
-				six_lock_should_sleep_fn should_sleep_fn, void *p,
-				unsigned long 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_type_ip_waiter(lock, type, &wait, should_sleep_fn, p, ip);
+	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_type_ip_waiter(lock, type, &wait, should_sleep_fn, p, _THIS_IP_);
+	return six_lock_ip_waiter(lock, type, &wait, should_sleep_fn, p, _THIS_IP_);
 }
 
-bool six_relock_ip_type(struct six_lock *lock, enum six_lock_type type,
-			unsigned seq, unsigned long 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_type(lock, type, seq, _THIS_IP_);
+	return six_relock_ip(lock, type, seq, _THIS_IP_);
 }
 
-void six_unlock_ip_type(struct six_lock *lock, enum six_lock_type type, unsigned long 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_type(lock, type, _THIS_IP_);
+	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_type(lock, SIX_LOCK_##type, ip);		\
+	return six_trylock_ip(lock, SIX_LOCK_##type, ip);		\
 }									\
 									\
 static inline bool six_trylock_##type(struct six_lock *lock)		\
 {									\
-	return six_trylock_ip_type(lock, SIX_LOCK_##type, _THIS_IP_);	\
+	return six_trylock_ip(lock, SIX_LOCK_##type, _THIS_IP_);	\
 }									\
 									\
 static inline int six_lock_ip_waiter_##type(struct six_lock *lock,	\
@@ -189,24 +332,24 @@ static inline int six_lock_ip_waiter_##type(struct six_lock *lock,	\
 			   six_lock_should_sleep_fn should_sleep_fn, void *p,\
 			   unsigned long ip)				\
 {									\
-	return six_lock_type_ip_waiter(lock, SIX_LOCK_##type, wait, should_sleep_fn, p, 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_type(lock, SIX_LOCK_##type, should_sleep_fn, p, 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_type(lock, SIX_LOCK_##type, seq, 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_type(lock, SIX_LOCK_##type, seq, _THIS_IP_);\
+	return six_relock_ip(lock, SIX_LOCK_##type, seq, _THIS_IP_);	\
 }									\
 									\
 static inline int six_lock_##type(struct six_lock *lock,		\
@@ -215,21 +358,14 @@ static inline int six_lock_##type(struct six_lock *lock,		\
 	return six_lock_ip_##type(lock, fn, p, _THIS_IP_);		\
 }									\
 									\
-static inline int six_lock_waiter_##type(struct six_lock *lock,		\
-			struct six_lock_waiter *wait,			\
-			six_lock_should_sleep_fn fn, void *p)		\
-{									\
-	return six_lock_ip_waiter_##type(lock, wait, fn, p, _THIS_IP_);	\
-}									\
-									\
 static inline void six_unlock_ip_##type(struct six_lock *lock, unsigned long ip)	\
 {									\
-	six_unlock_ip_type(lock, SIX_LOCK_##type, ip);			\
+	six_unlock_ip(lock, SIX_LOCK_##type, ip);			\
 }									\
 									\
 static inline void six_unlock_##type(struct six_lock *lock)		\
 {									\
-	six_unlock_ip_type(lock, SIX_LOCK_##type, _THIS_IP_);		\
+	six_unlock_ip(lock, SIX_LOCK_##type, _THIS_IP_);		\
 }
 
 __SIX_LOCK(read)