This just showed up at docker.

https://github.com/docker/docker/pull/15728

I can confirm https://github.com/docker/docker/pull/15728 solves the issue on docker-1.7.1.

As the fix was made for latest docker, i've tried to backport and seems to work very well, here's the patch:

diff --git a/graph/graph.go b/graph/graph.go
index f305622..28ab1f4 100644
--- a/graph/graph.go
+++ b/graph/graph.go
@@ -154,6 +154,10 @@ func (graph *Graph) Register(img *image.Image, layerData archive.ArchiveReader)
        // this doesn't mean Register is fully safe yet.
        graph.imageMutex.Lock(img.ID)
        defer graph.imageMutex.Unlock(img.ID)
+
+        if graph.Exists(img.ID) {
+               return nil
+       }

        defer func() {
                // If any error occurs, remove the new dir from the driver.
@@ -164,11 +168,6 @@ func (graph *Graph) Register(img *image.Image, layerData archive.ArchiveReader)
                }
        }()

-       // (This is a convenience to save time. Race conditions are taken care of by os.Rename)
-       if graph.Exists(img.ID) {
-               return fmt.Errorf("Image %s already exists", img.ID)
-       }
-
        // Ensure that the image root does not exist on the filesystem
        // when it is not registered in the graph.
        // This is common when you switch from one graph driver to another


After rebuild i couldn't reproduce the panic running the following script:

for image in openshift/nodejs-010-centos7 openshift/base-centos7 openshift/origin-sti-builder openshift/ruby-20-centos7 openshift/mysql-55-centos7 openshift/python-33-centos7 mesosphere/chronos:chronos-2.3.4-1.0.81.ubuntu1404-mesos-0.22.1-1.0.ubuntu1404 mesosphere/mesos-master:0.22.1-1.0.ubuntu1404 mesosphere/mesos-slave:0.22.1-1.0.ubuntu1404 mesosphere/marathon:v0.8.2-RC4 ; do     docker pull $image & done

Please update the pull request to give positive feedback.

With the comment3 steps, reproduced in docker-1.7.1-114.el7.x86_64, the docker daemon crashed
and verified in docker-1.7.1-115.el7.x86_64

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2015-1782.html