2017-03-16 23:18:50 -07:00
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/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _LINUX_SIX_H
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#define _LINUX_SIX_H
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2023-05-21 12:40:40 -07:00
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/**
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* DOC: SIX locks overview
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2017-03-16 23:18:50 -07:00
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*
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* Shared/intent/exclusive locks: sleepable read/write locks, like rw semaphores
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* but with an additional state: read/shared, intent, exclusive/write
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*
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* The purpose of the intent state is to allow for greater concurrency on tree
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* structures without deadlocking. In general, a read can't be upgraded to a
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* write lock without deadlocking, so an operation that updates multiple nodes
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* will have to take write locks for the full duration of the operation.
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*
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* But by adding an intent state, which is exclusive with other intent locks but
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* not with readers, we can take intent locks at the start of the operation,
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* and then take write locks only for the actual update to each individual
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* nodes, without deadlocking.
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*
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* Example usage:
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* six_lock_read(&foo->lock);
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* six_unlock_read(&foo->lock);
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*
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* An intent lock must be held before taking a write lock:
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* six_lock_intent(&foo->lock);
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* six_lock_write(&foo->lock);
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* six_unlock_write(&foo->lock);
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* six_unlock_intent(&foo->lock);
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*
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* Other operations:
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* six_trylock_read()
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* six_trylock_intent()
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* six_trylock_write()
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*
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* six_lock_downgrade() convert from intent to read
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* six_lock_tryupgrade() attempt to convert from read to intent, may fail
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*
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* There are also interfaces that take the lock type as an enum:
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*
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* six_lock_type(&foo->lock, SIX_LOCK_read);
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* six_trylock_convert(&foo->lock, SIX_LOCK_read, SIX_LOCK_intent)
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* six_lock_type(&foo->lock, SIX_LOCK_write);
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* six_unlock_type(&foo->lock, SIX_LOCK_write);
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* six_unlock_type(&foo->lock, SIX_LOCK_intent);
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*
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* Lock sequence numbers - unlock(), relock():
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*
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* Locks embed sequences numbers, which are incremented on write lock/unlock.
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* This allows locks to be dropped and the retaken iff the state they protect
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* hasn't changed; this makes it much easier to avoid holding locks while e.g.
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* doing IO or allocating memory.
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*
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* Example usage:
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* six_lock_read(&foo->lock);
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* u32 seq = six_lock_seq(&foo->lock);
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* six_unlock_read(&foo->lock);
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*
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* some_operation_that_may_block();
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*
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* if (six_relock_read(&foo->lock, seq)) { ... }
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*
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* If the relock operation succeeds, it is as if the lock was never unlocked.
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*
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* Reentrancy:
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*
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* Six locks are not by themselves reentrant, but have counters for both the
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* read and intent states that can be used to provide reentrancy by an upper
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* layer that tracks held locks. If a lock is known to already be held in the
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* read or intent state, six_lock_increment() can be used to bump the "lock
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* held in this state" counter, increasing the number of unlock calls that
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* will be required to fully unlock it.
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*
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* Example usage:
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* six_lock_read(&foo->lock);
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* six_lock_increment(&foo->lock, SIX_LOCK_read);
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* six_unlock_read(&foo->lock);
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* six_unlock_read(&foo->lock);
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* foo->lock is now fully unlocked.
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*
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* Since the intent state supercedes read, it's legal to increment the read
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* counter when holding an intent lock, but not the reverse.
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*
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* A lock may only be held once for write: six_lock_increment(.., SIX_LOCK_write)
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* is not legal.
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*
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* should_sleep_fn:
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*
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* There is a six_lock() variant that takes a function pointer that is called
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* immediately prior to schedule() when blocking, and may return an error to
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* abort.
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*
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* One possible use for this feature is when objects being locked are part of
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* a cache and may reused, and lock ordering is based on a property of the
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* object that will change when the object is reused - i.e. logical key order.
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*
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* If looking up an object in the cache may race with object reuse, and lock
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* ordering is required to prevent deadlock, object reuse may change the
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* correct lock order for that object and cause a deadlock. should_sleep_fn
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* can be used to check if the object is still the object we want and avoid
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* this deadlock.
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*
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* Wait list entry interface:
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*
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* There is a six_lock() variant, six_lock_waiter(), that takes a pointer to a
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* wait list entry. By embedding six_lock_waiter into another object, and by
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* traversing lock waitlists, it is then possible for an upper layer to
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* implement full cycle detection for deadlock avoidance.
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*
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* should_sleep_fn should be used for invoking the cycle detector, walking the
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* graph of held locks to check for a deadlock. The upper layer must track
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* held locks for each thread, and each thread's held locks must be reachable
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* from its six_lock_waiter object.
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*
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* six_lock_waiter() will add the wait object to the waitlist re-trying taking
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* the lock, and before calling should_sleep_fn, and the wait object will not
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* be removed from the waitlist until either the lock has been successfully
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* acquired, or we aborted because should_sleep_fn returned an error.
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*
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* Also, six_lock_waiter contains a timestamp, and waiters on a waitlist will
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* have timestamps in strictly ascending order - this is so the timestamp can
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* be used as a cursor for lock graph traverse.
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2017-03-16 23:18:50 -07:00
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*/
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#include <linux/lockdep.h>
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#include <linux/sched.h>
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#include <linux/types.h>
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enum six_lock_type {
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SIX_LOCK_read,
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SIX_LOCK_intent,
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SIX_LOCK_write,
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};
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struct six_lock {
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atomic_t state;
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u32 seq;
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unsigned intent_lock_recurse;
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struct task_struct *owner;
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2022-08-26 16:22:24 -07:00
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unsigned __percpu *readers;
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raw_spinlock_t wait_lock;
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struct list_head wait_list;
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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struct lockdep_map dep_map;
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#endif
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};
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2022-08-25 07:49:52 -07:00
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struct six_lock_waiter {
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struct list_head list;
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struct task_struct *task;
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enum six_lock_type lock_want;
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bool lock_acquired;
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2022-09-23 22:33:13 -07:00
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u64 start_time;
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};
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2017-03-16 23:18:50 -07:00
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typedef int (*six_lock_should_sleep_fn)(struct six_lock *lock, void *);
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void six_lock_exit(struct six_lock *lock);
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enum six_lock_init_flags {
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SIX_LOCK_INIT_PCPU = 1U << 0,
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};
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void __six_lock_init(struct six_lock *lock, const char *name,
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struct lock_class_key *key, enum six_lock_init_flags flags);
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2023-05-21 12:40:40 -07:00
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/**
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* six_lock_init - initialize a six lock
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* @lock: lock to initialize
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* @flags: optional flags, i.e. SIX_LOCK_INIT_PCPU
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*/
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#define six_lock_init(lock, flags) \
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do { \
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static struct lock_class_key __key; \
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\
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__six_lock_init((lock), #lock, &__key, flags); \
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} while (0)
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2023-05-21 12:40:40 -07:00
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/**
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* six_lock_seq - obtain current lock sequence number
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* @lock: six_lock to obtain sequence number for
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*
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* @lock should be held for read or intent, and not write
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*
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* By saving the lock sequence number, we can unlock @lock and then (typically
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* after some blocking operation) attempt to relock it: the relock will succeed
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* if the sequence number hasn't changed, meaning no write locks have been taken
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* and state corresponding to what @lock protects is still valid.
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*/
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static inline u32 six_lock_seq(const struct six_lock *lock)
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{
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return lock->seq;
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}
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bool six_trylock_ip(struct six_lock *lock, enum six_lock_type type, unsigned long ip);
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2023-05-21 12:40:40 -07:00
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/**
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* six_trylock_type - attempt to take a six lock without blocking
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* @lock: lock to take
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* @type: SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
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*
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* Return: true on success, false on failure.
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*/
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static inline bool six_trylock_type(struct six_lock *lock, enum six_lock_type type)
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{
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return six_trylock_ip(lock, type, _THIS_IP_);
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}
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int six_lock_ip_waiter(struct six_lock *lock, enum six_lock_type type,
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struct six_lock_waiter *wait,
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six_lock_should_sleep_fn should_sleep_fn, void *p,
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unsigned long ip);
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/**
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* six_lock_waiter - take a lock, with full waitlist interface
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* @lock: lock to take
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* @type: SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
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* @wait: pointer to wait object, which will be added to lock's waitlist
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* @should_sleep_fn: callback run after adding to waitlist, immediately prior
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* to scheduling
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* @p: passed through to @should_sleep_fn
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*
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* This is a convenience wrapper around six_lock_ip_waiter(), see that function
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* for full documentation.
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*
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* Return: 0 on success, or the return code from @should_sleep_fn on failure.
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*/
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static inline int six_lock_waiter(struct six_lock *lock, enum six_lock_type type,
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struct six_lock_waiter *wait,
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six_lock_should_sleep_fn should_sleep_fn, void *p)
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{
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return six_lock_ip_waiter(lock, type, wait, should_sleep_fn, p, _THIS_IP_);
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}
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2023-05-21 12:40:40 -07:00
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/**
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* six_lock_ip - take a six lock lock
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* @lock: lock to take
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* @type: SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
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* @should_sleep_fn: callback run after adding to waitlist, immediately prior
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* to scheduling
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* @p: passed through to @should_sleep_fn
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* @ip: ip parameter for lockdep/lockstat, i.e. _THIS_IP_
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*
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* Return: 0 on success, or the return code from @should_sleep_fn on failure.
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*/
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static inline int six_lock_ip(struct six_lock *lock, enum six_lock_type type,
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six_lock_should_sleep_fn should_sleep_fn, void *p,
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unsigned long ip)
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{
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struct six_lock_waiter wait;
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return six_lock_ip_waiter(lock, type, &wait, should_sleep_fn, p, ip);
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}
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2023-05-21 12:40:40 -07:00
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/**
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* six_lock_type - take a six lock lock
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* @lock: lock to take
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* @type: SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
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* @should_sleep_fn: callback run after adding to waitlist, immediately prior
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* to scheduling
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* @p: passed through to @should_sleep_fn
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*
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* Return: 0 on success, or the return code from @should_sleep_fn on failure.
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*/
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2023-05-20 18:44:30 -07:00
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static inline int six_lock_type(struct six_lock *lock, enum six_lock_type type,
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six_lock_should_sleep_fn should_sleep_fn, void *p)
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{
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struct six_lock_waiter wait;
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return six_lock_ip_waiter(lock, type, &wait, should_sleep_fn, p, _THIS_IP_);
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}
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bool six_relock_ip(struct six_lock *lock, enum six_lock_type type,
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unsigned seq, unsigned long ip);
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2023-05-21 12:40:40 -07:00
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/**
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* six_relock_type - attempt to re-take a lock that was held previously
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* @lock: lock to take
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* @type: SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
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* @seq: lock sequence number obtained from six_lock_seq() while lock was
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* held previously
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*
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* Return: true on success, false on failure.
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*/
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static inline bool six_relock_type(struct six_lock *lock, enum six_lock_type type,
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unsigned seq)
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{
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return six_relock_ip(lock, type, seq, _THIS_IP_);
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}
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void six_unlock_ip(struct six_lock *lock, enum six_lock_type type, unsigned long ip);
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2023-05-21 12:40:40 -07:00
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/**
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* six_unlock_type - drop a six lock
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* @lock: lock to unlock
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* @type: SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
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*
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* When a lock is held multiple times (because six_lock_incement()) was used),
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* this decrements the 'lock held' counter by one.
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*
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* For example:
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* six_lock_read(&foo->lock); read count 1
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* six_lock_increment(&foo->lock, SIX_LOCK_read); read count 2
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* six_lock_unlock(&foo->lock, SIX_LOCK_read); read count 1
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* six_lock_unlock(&foo->lock, SIX_LOCK_read); read count 0
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*/
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2023-05-20 18:44:30 -07:00
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static inline void six_unlock_type(struct six_lock *lock, enum six_lock_type type)
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{
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2023-05-21 12:40:40 -07:00
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six_unlock_ip(lock, type, _THIS_IP_);
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2023-05-20 18:44:30 -07:00
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}
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2017-03-16 23:18:50 -07:00
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#define __SIX_LOCK(type) \
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2023-05-20 18:44:30 -07:00
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static inline bool six_trylock_ip_##type(struct six_lock *lock, unsigned long ip)\
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{ \
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2023-05-21 12:40:40 -07:00
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return six_trylock_ip(lock, SIX_LOCK_##type, ip); \
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2023-05-20 18:44:30 -07:00
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} \
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2023-02-04 17:38:43 -07:00
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\
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static inline bool six_trylock_##type(struct six_lock *lock) \
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{ \
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2023-05-21 12:40:40 -07:00
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return six_trylock_ip(lock, SIX_LOCK_##type, _THIS_IP_); \
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2023-05-20 18:44:30 -07:00
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} \
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\
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static inline int six_lock_ip_waiter_##type(struct six_lock *lock, \
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struct six_lock_waiter *wait, \
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six_lock_should_sleep_fn should_sleep_fn, void *p,\
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unsigned long ip) \
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{ \
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2023-05-21 12:40:40 -07:00
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return six_lock_ip_waiter(lock, SIX_LOCK_##type, wait, should_sleep_fn, p, ip);\
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2023-05-20 18:44:30 -07:00
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} \
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\
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static inline int six_lock_ip_##type(struct six_lock *lock, \
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six_lock_should_sleep_fn should_sleep_fn, void *p, \
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unsigned long ip) \
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{ \
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2023-05-21 12:40:40 -07:00
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return six_lock_ip(lock, SIX_LOCK_##type, should_sleep_fn, p, ip);\
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2023-05-20 18:44:30 -07:00
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} \
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\
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static inline bool six_relock_ip_##type(struct six_lock *lock, u32 seq, unsigned long ip)\
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{ \
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2023-05-21 12:40:40 -07:00
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return six_relock_ip(lock, SIX_LOCK_##type, seq, ip); \
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2023-02-04 17:38:43 -07:00
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} \
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2023-05-20 18:44:30 -07:00
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\
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2023-02-04 17:38:43 -07:00
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static inline bool six_relock_##type(struct six_lock *lock, u32 seq) \
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{ \
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2023-05-21 12:40:40 -07:00
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return six_relock_ip(lock, SIX_LOCK_##type, seq, _THIS_IP_); \
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2023-02-04 17:38:43 -07:00
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} \
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2023-05-20 18:44:30 -07:00
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\
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2023-02-04 17:38:43 -07:00
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static inline int six_lock_##type(struct six_lock *lock, \
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six_lock_should_sleep_fn fn, void *p)\
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{ \
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return six_lock_ip_##type(lock, fn, p, _THIS_IP_); \
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} \
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2023-05-20 18:44:30 -07:00
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\
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static inline void six_unlock_ip_##type(struct six_lock *lock, unsigned long ip) \
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{ \
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2023-05-21 12:40:40 -07:00
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six_unlock_ip(lock, SIX_LOCK_##type, ip); \
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2023-05-20 18:44:30 -07:00
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} \
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\
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2023-02-04 17:38:43 -07:00
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static inline void six_unlock_##type(struct six_lock *lock) \
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{ \
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2023-05-21 12:40:40 -07:00
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six_unlock_ip(lock, SIX_LOCK_##type, _THIS_IP_); \
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2023-02-04 17:38:43 -07:00
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}
|
2017-03-16 23:18:50 -07:00
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__SIX_LOCK(read)
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__SIX_LOCK(intent)
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__SIX_LOCK(write)
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|
#undef __SIX_LOCK
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void six_lock_downgrade(struct six_lock *);
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|
bool six_lock_tryupgrade(struct six_lock *);
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|
|
bool six_trylock_convert(struct six_lock *, enum six_lock_type,
|
|
|
|
enum six_lock_type);
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|
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|
void six_lock_increment(struct six_lock *, enum six_lock_type);
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|
void six_lock_wakeup_all(struct six_lock *);
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|
struct six_lock_count {
|
2022-08-21 20:08:53 -07:00
|
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|
unsigned n[3];
|
2017-03-16 23:18:50 -07:00
|
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|
};
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struct six_lock_count six_lock_counts(struct six_lock *);
|
2023-05-20 17:40:08 -07:00
|
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|
void six_lock_readers_add(struct six_lock *, int);
|
2017-03-16 23:18:50 -07:00
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#endif /* _LINUX_SIX_H */
|