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Date: Wed, 14 Mar 2012 22:19:08 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Sasha Levin <levinsasha928@gmail.com>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        davej@redhat.com, Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH] vfs namespace: Don't assume mount namespace has valid
 root
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On Thu, Mar 15, 2012 at 12:00:52AM +0200, Sasha Levin wrote:

> I'm running 3.3.0-rc6-next-20120309-sasha-00002-g8fd19a0.
> 
> I found that if I revert f8b88187 ("brlocks/lglocks: cleanups") it
> seems that the problem is gone.

... along with this piece of offending commit:
@@ -2249,10 +2250,9 @@ static struct mnt_namespace *dup_mnt_ns(struct mnt_namespace *mnt_ns,
                kfree(new_ns);
                return ERR_PTR(-ENOMEM);
        }
-       new_ns->root = new;
-       br_write_lock(vfsmount_lock);
+       br_write_lock(&vfsmount_lock);
        list_add_tail(&new_ns->list, &new->mnt_list);
-       br_write_unlock(vfsmount_lock);
+       br_write_unlock(&vfsmount_lock);

        /*
         * Second pass: switch the tsk->fs->* elements and mark new vfsmounts
@@ -2416,9 +2416,9 @@ bool is_path_reachable(struct mount *mnt, struct dentry *dentry,

IOW, Andi has hit delete one too many times...