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Diffstat (limited to 'fs/bcachefs/six.h')
| -rw-r--r-- | fs/bcachefs/six.h | 393 |
1 files changed, 393 insertions, 0 deletions
diff --git a/fs/bcachefs/six.h b/fs/bcachefs/six.h new file mode 100644 index 0000000000..4c268b0b83 --- /dev/null +++ b/fs/bcachefs/six.h @@ -0,0 +1,393 @@ +/* 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 thte 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 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> +#include <linux/sched.h> +#include <linux/types.h> + +#ifdef CONFIG_SIX_LOCK_SPIN_ON_OWNER +#include <linux/osq_lock.h> +#endif + +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; +#ifdef CONFIG_SIX_LOCK_SPIN_ON_OWNER + struct optimistic_spin_queue osq; +#endif + 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 */ |