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author | Kent Overstreet <kent.overstreet@linux.dev> | 2023-05-21 15:40:40 -0400 |
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committer | Kent Overstreet <kent.overstreet@linux.dev> | 2023-10-22 17:10:02 -0400 |
commit | 91d16f16d0fd4b6eb8503068ea7f6ad8305e32db (patch) | |
tree | 277706ee9c5ca1b52f3a10612b26687980698f92 /fs/bcachefs/six.h | |
parent | 1fb4fe63178881a0ac043a5c05288d9fff85d6b8 (diff) | |
download | linux-91d16f16d0fd4b6eb8503068ea7f6ad8305e32db.tar.gz linux-91d16f16d0fd4b6eb8503068ea7f6ad8305e32db.tar.bz2 linux-91d16f16d0fd4b6eb8503068ea7f6ad8305e32db.zip |
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>
Diffstat (limited to 'fs/bcachefs/six.h')
-rw-r--r-- | fs/bcachefs/six.h | 298 |
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) |