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layer: protect from same-name races

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As pointed out by Tonis, there's a race between ReleaseRWLayer()
and GetRWLayer():

```
----- goroutine 1 -----               ----- goroutine 2 -----
ReleaseRWLayer()
  m := ls.mounts[l.Name()]
  ...
  m.deleteReference(l)
  m.hasReferences()
  ...                                 GetRWLayer()
  ...                                   mount := ls.mounts[id]
  ls.driver.Remove(m.mountID)
  ls.store.RemoveMount(m.name)          return mount.getReference()
  delete(ls.mounts, m.Name())
-----------------------               -----------------------
```

When something like this happens, GetRWLayer will return
an RWLayer without a storage. Oops.

There might be more races like this, and it seems the best
solution is to lock by layer id/name by using pkg/locker.

With this in place, name collision could not happen, so remove
the part of previous commit that protected against it in
CreateRWLayer (temporary nil assigmment and associated rollback).

So, now we have
* layerStore.mountL sync.Mutex to protect layerStore.mount map[]
  (against concurrent access);
* mountedLayer's embedded `sync.Mutex` to protect its references map[];
* layerStore.layerL (which I haven't touched);
* per-id locker, to avoid name conflicts and concurrent operations
  on the same rw layer.

The whole rig seems to look more readable now (mutexes use is
straightforward, no nested locks).

Reported-by: Tonis Tiigi <tonistiigi@gmail.com>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit af433dd200)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
This commit is contained in:
Kir Kolyshkin 2019-05-20 14:46:54 -07:00 committed by Sebastiaan van Stijn
parent 9eeb2b5ef0
commit 936432326a
No known key found for this signature in database
GPG key ID: 76698F39D527CE8C
1 changed files with 25 additions and 17 deletions
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42
layer/layer_store.go
View file

@ -10,6 +10,7 @@ import (
"github.com/docker/distribution"
"github.com/docker/docker/daemon/graphdriver"
"github.com/docker/docker/pkg/idtools"
"github.com/docker/docker/pkg/locker"
"github.com/docker/docker/pkg/plugingetter"
"github.com/docker/docker/pkg/stringid"
"github.com/docker/docker/pkg/system"
@ -36,7 +37,11 @@ type layerStore struct {
mounts map[string]*mountedLayer
mountL sync.Mutex
os string
// protect *RWLayer() methods from operating on the same name/id
locker *locker.Locker
os string
}
// StoreOptions are the options used to create a new Store instance
@ -92,6 +97,7 @@ func newStoreFromGraphDriver(root string, driver graphdriver.Driver, os string)
driver: driver,
layerMap: map[ChainID]*roLayer{},
mounts: map[string]*mountedLayer{},
locker: locker.New(),
useTarSplit: !caps.ReproducesExactDiffs,
os: os,
}
@ -191,7 +197,7 @@ func (ls *layerStore) loadLayer(layer ChainID) (*roLayer, error) {
func (ls *layerStore) loadMount(mount string) error {
ls.mountL.Lock()
defer ls.mountL.Unlock()
if m := ls.mounts[mount]; m != nil {
if _, ok := ls.mounts[mount]; ok {
return nil
}
@ -492,21 +498,15 @@ func (ls *layerStore) CreateRWLayer(name string, parent ChainID, opts *CreateRWL
initFunc = opts.InitFunc
}
ls.locker.Lock(name)
defer ls.locker.Unlock(name)
ls.mountL.Lock()
if _, ok := ls.mounts[name]; ok {
ls.mountL.Unlock()
_, ok := ls.mounts[name]
ls.mountL.Unlock()
if ok {
return nil, ErrMountNameConflict
}
// Avoid name collision by temporary assigning nil
ls.mounts[name] = nil
ls.mountL.Unlock()
defer func() {
if err != nil {
ls.mountL.Lock()
delete(ls.mounts, name)
ls.mountL.Unlock()
}
}()
var pid string
var p *roLayer
@ -558,6 +558,9 @@ func (ls *layerStore) CreateRWLayer(name string, parent ChainID, opts *CreateRWL
}
func (ls *layerStore) GetRWLayer(id string) (RWLayer, error) {
ls.locker.Lock(id)
defer ls.locker.Unlock(id)
ls.mountL.Lock()
mount := ls.mounts[id]
ls.mountL.Unlock()
@ -570,8 +573,9 @@ func (ls *layerStore) GetRWLayer(id string) (RWLayer, error) {
func (ls *layerStore) GetMountID(id string) (string, error) {
ls.mountL.Lock()
defer ls.mountL.Unlock()
mount := ls.mounts[id]
ls.mountL.Unlock()
if mount == nil {
return "", ErrMountDoesNotExist
}
@ -581,8 +585,12 @@ func (ls *layerStore) GetMountID(id string) (string, error) {
}
func (ls *layerStore) ReleaseRWLayer(l RWLayer) ([]Metadata, error) {
name := l.Name()
ls.locker.Lock(name)
defer ls.locker.Unlock(name)
ls.mountL.Lock()
m := ls.mounts[l.Name()]
m := ls.mounts[name]
ls.mountL.Unlock()
if m == nil {
return []Metadata{}, nil
@ -617,7 +625,7 @@ func (ls *layerStore) ReleaseRWLayer(l RWLayer) ([]Metadata, error) {
}
ls.mountL.Lock()
delete(ls.mounts, m.Name())
delete(ls.mounts, name)
ls.mountL.Unlock()
ls.layerL.Lock()