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https://github.com/moby/moby.git
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fe66fdd18c
Uses finer grained locking so that each volume name gets its own lock rather than only being protected by the global lock, which itself needs to be unlocked during cetain operations (`create` especially`) Signed-off-by: Brian Goff <cpuguy83@gmail.com>
111 lines
2.6 KiB
Go
111 lines
2.6 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
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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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waiters uint32
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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.AddUint32(&l.waiters, 1)
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}
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// dec decrements the number of waiters wating on the lock
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func (l *lockCtr) dec() {
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atomic.AddUint32(&l.waiters, ^uint32(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() uint32 {
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return atomic.LoadUint32(&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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