devmapper: storage-opt override for udev sync

This provides an override for forcing the daemon to still attempt
running the devicemapper driver even when udev sync is not supported.

Intended to be a very clear impairment for those choosing to use it. If
udev sync is false, there will still be an error in the daemon logs,
even when the override is in place. The docs have an explicit WARNING.

Including link to the docs for users that encounter this daemon error
during an upgrade.

Signed-off-by: Vincent Batts <vbatts@redhat.com>

daemon/graphdriver/devmapper/README.md

@@ -186,7 +186,7 @@ Here is the list of supported options:
     can be achieved by zeroing the first 4k to indicate empty
     metadata, like this:
 
-    ``dd if=/dev/zero of=$metadata_dev bs=4096 count=1```
+    ``dd if=/dev/zero of=$metadata_dev bs=4096 count=1``
 
     Example use:
 
@@ -216,3 +216,39 @@ Here is the list of supported options:
     Example use:
 
     ``docker -d --storage-opt dm.blkdiscard=false``
+
+ *  `dm.override_udev_sync_check`
+
+    Overrides the `udev` synchronization checks between `devicemapper` and `udev`.
+    `udev` is the device manager for the Linux kernel.
+
+    To view the `udev` sync support of a Docker daemon that is using the
+    `devicemapper` driver, run:
+
+        $ docker info
+	[...]
+	 Udev Sync Supported: true
+	[...]
+
+    When `udev` sync support is `true`, then `devicemapper` and udev can
+    coordinate the activation and deactivation of devices for containers.
+
+    When `udev` sync support is `false`, a race condition occurs between
+    the`devicemapper` and `udev` during create and cleanup. The race condition
+    results in errors and failures. (For information on these failures, see
+    [docker#4036](https://github.com/docker/docker/issues/4036))
+
+    To allow the `docker` daemon to start, regardless of `udev` sync not being
+    supported, set `dm.override_udev_sync_check` to true:
+
+        $ docker -d --storage-opt dm.override_udev_sync_check=true
+
+    When this value is `true`, the  `devicemapper` continues and simply warns
+    you the errors are happening.
+
+    > **Note**: The ideal is to pursue a `docker` daemon and environment that
+    > does support synchronizing with `udev`. For further discussion on this
+    > topic, see [docker#4036](https://github.com/docker/docker/issues/4036).
+    > Otherwise, set this flag for migrating existing Docker daemons to a
+    > daemon with a supported environment.
+

daemon/graphdriver/devmapper/deviceset.go

@@ -31,6 +31,7 @@ var (
 	DefaultMetaDataLoopbackSize int64  = 2 * 1024 * 1024 * 1024
 	DefaultBaseFsSize           uint64 = 10 * 1024 * 1024 * 1024
 	DefaultThinpBlockSize       uint32 = 128 // 64K = 128 512b sectors
+	DefaultUdevSyncOverride     bool   = false
 	MaxDeviceId                 int    = 0xffffff // 24 bit, pool limit
 	DeviceIdMapSz               int    = (MaxDeviceId + 1) / 8
 )
@@ -97,6 +98,7 @@ type DeviceSet struct {
 	thinpBlockSize        uint32
 	thinPoolDevice        string
 	Transaction           `json:"-"`
+	overrideUdevSyncCheck bool
 }
 
 type DiskUsage struct {
@@ -963,9 +965,11 @@ func (devices *DeviceSet) initDevmapper(doInit bool) error {
 
 	// https://github.com/docker/docker/issues/4036
 	if supported := devicemapper.UdevSetSyncSupport(true); !supported {
-		logrus.Errorf("Udev sync is not supported. This will lead to unexpected behavior, data loss and errors")
+		logrus.Errorf("Udev sync is not supported. This will lead to unexpected behavior, data loss and errors. For more information, see https://docs.docker.com/reference/commandline/cli/#daemon-storage-driver-option")
+		if !devices.overrideUdevSyncCheck {
 			return graphdriver.ErrNotSupported
 		}
+	}
 
 	if err := os.MkdirAll(devices.metadataDir(), 0700); err != nil && !os.IsExist(err) {
 		return err
@@ -1661,6 +1665,7 @@ func NewDeviceSet(root string, doInit bool, options []string) (*DeviceSet, error
 		dataLoopbackSize:      DefaultDataLoopbackSize,
 		metaDataLoopbackSize:  DefaultMetaDataLoopbackSize,
 		baseFsSize:            DefaultBaseFsSize,
+		overrideUdevSyncCheck: DefaultUdevSyncOverride,
 		filesystem:            "ext4",
 		doBlkDiscard:          true,
 		thinpBlockSize:        DefaultThinpBlockSize,
@@ -1721,6 +1726,11 @@ func NewDeviceSet(root string, doInit bool, options []string) (*DeviceSet, error
 			}
 			// convert to 512b sectors
 			devices.thinpBlockSize = uint32(size) >> 9
+		case "dm.override_udev_sync_check":
+			devices.overrideUdevSyncCheck, err = strconv.ParseBool(val)
+			if err != nil {
+				return nil, err
+			}
 		default:
 			return nil, fmt.Errorf("Unknown option %s\n", key)
 		}

daemon/graphdriver/devmapper/devmapper_test.go

@@ -13,6 +13,7 @@ func init() {
 	DefaultDataLoopbackSize = 300 * 1024 * 1024
 	DefaultMetaDataLoopbackSize = 200 * 1024 * 1024
 	DefaultBaseFsSize = 300 * 1024 * 1024
+	DefaultUdevSyncOverride = true
 	if err := graphtest.InitLoopbacks(); err != nil {
 		panic(err)
 	}

docs/sources/reference/commandline/cli.md

@@ -370,6 +370,41 @@ Currently supported options are:
 
         $ docker -d --storage-opt dm.blkdiscard=false
 
+ *  `dm.override_udev_sync_check`
+
+    Overrides the `udev` synchronization checks between `devicemapper` and `udev`.
+    `udev` is the device manager for the Linux kernel.
+
+    To view the `udev` sync support of a Docker daemon that is using the
+    `devicemapper` driver, run:
+
+        $ docker info
+	[...]
+	 Udev Sync Supported: true
+	[...]
+
+    When `udev` sync support is `true`, then `devicemapper` and udev can
+    coordinate the activation and deactivation of devices for containers.
+
+    When `udev` sync support is `false`, a race condition occurs between
+    the`devicemapper` and `udev` during create and cleanup. The race condition
+    results in errors and failures. (For information on these failures, see
+    [docker#4036](https://github.com/docker/docker/issues/4036))
+
+    To allow the `docker` daemon to start, regardless of `udev` sync not being
+    supported, set `dm.override_udev_sync_check` to true:
+
+        $ docker -d --storage-opt dm.override_udev_sync_check=true
+
+    When this value is `true`, the  `devicemapper` continues and simply warns
+    you the errors are happening.
+
+    > **Note**: The ideal is to pursue a `docker` daemon and environment that
+    > does support synchronizing with `udev`. For further discussion on this
+    > topic, see [docker#4036](https://github.com/docker/docker/issues/4036).
+    > Otherwise, set this flag for migrating existing Docker daemons to a
+    > daemon with a supported environment.
+
 ### Docker exec-driver option
 
 The Docker daemon uses a specifically built `libcontainer` execution driver as its