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moby--moby/pkg/locker
Brian Goff fe66fdd18c Fix potential races in the volume store
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>
2015-11-04 16:59:47 -05:00
..
locker.go Fix potential races in the volume store 2015-11-04 16:59:47 -05:00
locker_test.go Fix potential races in the volume store 2015-11-04 16:59:47 -05:00
README.md Fix potential races in the volume store 2015-11-04 16:59:47 -05:00

Locker

locker provides a mechanism for creating finer-grained locking to help free up more global locks to handle other tasks.

The implementation looks close to a sync.Mutex, however the user must provide a reference to use to refer to the underlying lock when locking and unlocking, and unlock may generate an error.

If a lock with a given name does not exist when Lock is called, one is created. Lock references are automatically cleaned up on Unlock if nothing else is waiting for the lock.

Usage

package important

import (
	"sync"
	"time"

	"github.com/docker/docker/pkg/locker"
)

type important struct {
	locks *locker.Locker
	data  map[string]interface{}
	mu    sync.Mutex
}

func (i *important) Get(name string) interface{} {
	i.locks.Lock(name)
	defer i.locks.Unlock(name)
	return data[name]
}

func (i *important) Create(name string, data interface{}) {
	i.locks.Lock(name)
	defer i.locks.Unlock(name)

	i.createImporatant(data)

	s.mu.Lock()
	i.data[name] = data
	s.mu.Unlock()
}

func (i *important) createImportant(data interface{}) {
	time.Sleep(10 * time.Second)
}

For functions dealing with a given name, always lock at the beginning of the function (or before doing anything with the underlying state), this ensures any other function that is dealing with the same name will block.

When needing to modify the underlying data, use the global lock to ensure nothing else is modfying it at the same time. Since name lock is already in place, no reads will occur while the modification is being performed.