112 lines
2.7 KiB
Go
112 lines
2.7 KiB
Go
/*
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Package locker provides a mechanism for creating finer-grained locking to help
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free up more global locks to handle other tasks.
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The implementation looks close to a sync.Mutex, however the user must provide a
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reference to use to refer to the underlying lock when locking and unlocking,
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and unlock may generate an error.
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If a lock with a given name does not exist when `Lock` is called, one is
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created.
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Lock references are automatically cleaned up on `Unlock` if nothing else is
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waiting for the lock.
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*/
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package locker // import "github.com/docker/docker/pkg/locker"
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import (
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"errors"
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"sync"
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"sync/atomic"
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)
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// ErrNoSuchLock is returned when the requested lock does not exist
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var ErrNoSuchLock = errors.New("no such lock")
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// Locker provides a locking mechanism based on the passed in reference name
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type Locker struct {
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mu sync.Mutex
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locks map[string]*lockCtr
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}
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// lockCtr is used by Locker to represent a lock with a given name.
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type lockCtr struct {
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mu sync.Mutex
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// waiters is the number of waiters waiting to acquire the lock
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// this is int32 instead of uint32 so we can add `-1` in `dec()`
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waiters int32
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}
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// inc increments the number of waiters waiting for the lock
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func (l *lockCtr) inc() {
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atomic.AddInt32(&l.waiters, 1)
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}
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// dec decrements the number of waiters waiting on the lock
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func (l *lockCtr) dec() {
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atomic.AddInt32(&l.waiters, -1)
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}
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// count gets the current number of waiters
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func (l *lockCtr) count() int32 {
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return atomic.LoadInt32(&l.waiters)
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}
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// Lock locks the mutex
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func (l *lockCtr) Lock() {
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l.mu.Lock()
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}
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// Unlock unlocks the mutex
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func (l *lockCtr) Unlock() {
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l.mu.Unlock()
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}
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// New creates a new Locker
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func New() *Locker {
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return &Locker{
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locks: make(map[string]*lockCtr),
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}
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}
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// Lock locks a mutex with the given name. If it doesn't exist, one is created
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func (l *Locker) Lock(name string) {
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l.mu.Lock()
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if l.locks == nil {
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l.locks = make(map[string]*lockCtr)
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}
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nameLock, exists := l.locks[name]
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if !exists {
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nameLock = &lockCtr{}
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l.locks[name] = nameLock
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}
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// increment the nameLock waiters while inside the main mutex
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// this makes sure that the lock isn't deleted if `Lock` and `Unlock` are called concurrently
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nameLock.inc()
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l.mu.Unlock()
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// Lock the nameLock outside the main mutex so we don't block other operations
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// once locked then we can decrement the number of waiters for this lock
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nameLock.Lock()
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nameLock.dec()
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}
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// Unlock unlocks the mutex with the given name
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// If the given lock is not being waited on by any other callers, it is deleted
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func (l *Locker) Unlock(name string) error {
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l.mu.Lock()
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nameLock, exists := l.locks[name]
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if !exists {
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l.mu.Unlock()
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return ErrNoSuchLock
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}
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if nameLock.count() == 0 {
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delete(l.locks, name)
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}
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nameLock.Unlock()
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l.mu.Unlock()
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return nil
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}
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