Merge pull request #11412 from vbatts/vbatts-dm_sync_is_required

devmapper: udev sync is a requirement

docs/sources/reference/commandline/cli.md

@@ -376,6 +376,41 @@ 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