ceph: document unlocked d_parent accesses

For the most part we don't care about racing with rename when directing
MDS requests; either the old or new parent is fine.  Document that, and
do some minor cleanup.

Reviewed-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Sage Weil <sage@newdream.net>

2 files changed, 11 insertions, 4 deletions

diff --git a/fs/ceph/dir.c b/fs/ceph/dir.c
index 7263f825d426..852ff8600ac9 100644
--- a/fs/ceph/dir.c
+++ b/fs/ceph/dir.c
@@ -506,7 +506,7 @@ int ceph_handle_snapdir(struct ceph_mds_request *req,
                         struct dentry *dentry, int err)
 {
         struct ceph_fs_client *fsc = ceph_sb_to_client(dentry->d_sb);
-        struct inode *parent = dentry->d_parent->d_inode;
+        struct inode *parent = dentry->d_parent->d_inode; /* we hold i_mutex */
 
         /* .snap dir? */
         if (err == -ENOENT &&
diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index 66a8939cc518..fee028b5332e 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -621,6 +621,12 @@ static void __unregister_request(struct ceph_mds_client *mdsc,
  */
 struct dentry *get_nonsnap_parent(struct dentry *dentry)
 {
+        /*
+         * we don't need to worry about protecting the d_parent access
+         * here because we never renaming inside the snapped namespace
+         * except to resplice to another snapdir, and either the old or new
+         * result is a valid result.
+         */
         while (!IS_ROOT(dentry) && ceph_snap(dentry->d_inode) != CEPH_NOSNAP)
                 dentry = dentry->d_parent;
         return dentry;
@@ -656,7 +662,9 @@ static int __choose_mds(struct ceph_mds_client *mdsc,
         if (req->r_inode) {
                 inode = req->r_inode;
         } else if (req->r_dentry) {
-                struct inode *dir = req->r_dentry->d_parent->d_inode;
+                /* ignore race with rename; old or new d_parent is okay */
+                struct dentry *parent = req->r_dentry->d_parent;
+                struct inode *dir = parent->d_inode;
 
                 if (dir->i_sb != mdsc->fsc->sb) {
                         /* not this fs! */
@@ -664,8 +672,7 @@ static int __choose_mds(struct ceph_mds_client *mdsc,
                 } else if (ceph_snap(dir) != CEPH_NOSNAP) {
                         /* direct snapped/virtual snapdir requests
                          * based on parent dir inode */
-                        struct dentry *dn =
-                                get_nonsnap_parent(req->r_dentry->d_parent);
+                        struct dentry *dn = get_nonsnap_parent(parent);
                         inode = dn->d_inode;
                         dout("__choose_mds using nonsnap parent %p\n", inode);
                 } else if (req->r_dentry->d_inode) {