ceph: handle racing calls to ceph_init_dentry

The ->lookup() and prepopulate_readdir() callers are working with unhashed
dentries, so we don't have to worry.  The export.c callers, though, need
to initialize something they got back from d_obtain_alias() and are
potentially racing with other callers.  Make sure we don't return unless
the dentry is properly initialized (by us or someone else).

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Reviewed-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Sage Weil <sage@newdream.net>

1 files changed, 12 insertions, 9 deletions

diff --git a/fs/ceph/dir.c b/fs/ceph/dir.c
index f39a409db0ea..883c9546111d 100644
--- a/fs/ceph/dir.c
+++ b/fs/ceph/dir.c
@@ -40,14 +40,6 @@ int ceph_init_dentry(struct dentry *dentry)
         if (dentry->d_fsdata)
                 return 0;
 
-        if (dentry->d_parent == NULL ||   /* nfs fh_to_dentry */
-            ceph_snap(dentry->d_parent->d_inode) == CEPH_NOSNAP)
-                d_set_d_op(dentry, &ceph_dentry_ops);
-        else if (ceph_snap(dentry->d_parent->d_inode) == CEPH_SNAPDIR)
-                d_set_d_op(dentry, &ceph_snapdir_dentry_ops);
-        else
-                d_set_d_op(dentry, &ceph_snap_dentry_ops);
-
         di = kmem_cache_alloc(ceph_dentry_cachep, GFP_NOFS | __GFP_ZERO);
         if (!di)
                 return -ENOMEM;          /* oh well */
@@ -58,10 +50,21 @@ int ceph_init_dentry(struct dentry *dentry)
                 kmem_cache_free(ceph_dentry_cachep, di);
                 goto out_unlock;
         }
+
+        if (dentry->d_parent == NULL ||   /* nfs fh_to_dentry */
+            ceph_snap(dentry->d_parent->d_inode) == CEPH_NOSNAP)
+                d_set_d_op(dentry, &ceph_dentry_ops);
+        else if (ceph_snap(dentry->d_parent->d_inode) == CEPH_SNAPDIR)
+                d_set_d_op(dentry, &ceph_snapdir_dentry_ops);
+        else
+                d_set_d_op(dentry, &ceph_snap_dentry_ops);
+
         di->dentry = dentry;
         di->lease_session = NULL;
-        dentry->d_fsdata = di;
         dentry->d_time = jiffies;
+        /* avoid reordering d_fsdata setup so that the check above is safe */
+        smp_mb();
+        dentry->d_fsdata = di;
         ceph_dentry_lru_add(dentry);
 out_unlock:
         spin_unlock(&dentry->d_lock);