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/* 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 */