fs/9p: Prevent parallel rename when doing fid_lookup

During fid lookup we need to make sure that the dentry->d_parent doesn't
change so that we can safely walk the parent dentries. To ensure that
we need to prevent cross directory rename during fid_lookup. Add a
per superblock rename_sem rw_semaphore to prevent parallel fid lookup and
rename.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

1 files changed, 78 insertions, 36 deletions

diff --git a/fs/9p/fid.c b/fs/9p/fid.c
index 5d6cfcbf73e7..358563689064 100644
--- a/fs/9p/fid.c
+++ b/fs/9p/fid.c
@@ -97,6 +97,34 @@ static struct p9_fid *v9fs_fid_find(struct dentry *dentry, u32 uid, int any)
         return ret;
 }
 
+/*
+ * We need to hold v9ses->rename_sem as long as we hold references
+ * to returned path array. Array element contain pointers to
+ * dentry names.
+ */
+static int build_path_from_dentry(struct v9fs_session_info *v9ses,
+                                  struct dentry *dentry, char ***names)
+{
+        int n = 0, i;
+        char **wnames;
+        struct dentry *ds;
+
+        for (ds = dentry; !IS_ROOT(ds); ds = ds->d_parent)
+                n++;
+
+        wnames = kmalloc(sizeof(char *) * n, GFP_KERNEL);
+        if (!wnames)
+                goto err_out;
+
+        for (ds = dentry, i = (n-1); i >= 0; i--, ds = ds->d_parent)
+                wnames[i] = (char  *)ds->d_name.name;
+
+        *names = wnames;
+        return n;
+err_out:
+        return -ENOMEM;
+}
+
 /**
  * v9fs_fid_lookup - lookup for a fid, try to walk if not found
  * @dentry: dentry to look for fid in
@@ -112,7 +140,7 @@ struct p9_fid *v9fs_fid_lookup(struct dentry *dentry)
         int i, n, l, clone, any, access;
         u32 uid;
         struct p9_fid *fid, *old_fid = NULL;
-        struct dentry *d, *ds;
+        struct dentry *ds;
         struct v9fs_session_info *v9ses;
         char **wnames, *uname;
 
@@ -139,50 +167,62 @@ struct p9_fid *v9fs_fid_lookup(struct dentry *dentry)
         fid = v9fs_fid_find(dentry, uid, any);
         if (fid)
                 return fid;
-
+        /*
+         * we don't have a matching fid. To do a TWALK we need
+         * parent fid. We need to prevent rename when we want to
+         * look at the parent.
+         */
+        down_read(&v9ses->rename_sem);
         ds = dentry->d_parent;
         fid = v9fs_fid_find(ds, uid, any);
-        if (!fid) { /* walk from the root */
-                n = 0;
-                for (ds = dentry; !IS_ROOT(ds); ds = ds->d_parent)
-                        n++;
-
-                fid = v9fs_fid_find(ds, uid, any);
-                if (!fid) { /* the user is not attached to the fs yet */
-                        if (access == V9FS_ACCESS_SINGLE)
-                                return ERR_PTR(-EPERM);
-
-                        if (v9fs_proto_dotu(v9ses) ||
-                                v9fs_proto_dotl(v9ses))
-                                uname = NULL;
-                        else
-                                uname = v9ses->uname;
+        if (fid) {
+                /* Found the parent fid do a lookup with that */
+                fid = p9_client_walk(fid, 1, (char **)&dentry->d_name.name, 1);
+                goto fid_out;
+        }
+        up_read(&v9ses->rename_sem);
 
-                        fid = p9_client_attach(v9ses->clnt, NULL, uname, uid,
-                                v9ses->aname);
+        /* start from the root and try to do a lookup */
+        fid = v9fs_fid_find(dentry->d_sb->s_root, uid, any);
+        if (!fid) {
+                /* the user is not attached to the fs yet */
+                if (access == V9FS_ACCESS_SINGLE)
+                        return ERR_PTR(-EPERM);
 
-                        if (IS_ERR(fid))
-                                return fid;
+                if (v9fs_proto_dotu(v9ses) || v9fs_proto_dotl(v9ses))
+                                uname = NULL;
+                else
+                        uname = v9ses->uname;
 
-                        v9fs_fid_add(ds, fid);
-                }
-        } else /* walk from the parent */
-                n = 1;
+                fid = p9_client_attach(v9ses->clnt, NULL, uname, uid,
+                                       v9ses->aname);
+                if (IS_ERR(fid))
+                        return fid;
 
-        if (ds == dentry)
+                v9fs_fid_add(dentry->d_sb->s_root, fid);
+        }
+        /* If we are root ourself just return that */
+        if (dentry->d_sb->s_root == dentry)
                 return fid;
-
-        wnames = kmalloc(sizeof(char *) * n, GFP_KERNEL);
-        if (!wnames)
-                return ERR_PTR(-ENOMEM);
-
-        for (d = dentry, i = (n-1); i >= 0; i--, d = d->d_parent)
-                wnames[i] = (char *) d->d_name.name;
-
+        /*
+         * Do a multipath walk with attached root.
+         * When walking parent we need to make sure we
+         * don't have a parallel rename happening
+         */
+        down_read(&v9ses->rename_sem);
+        n  = build_path_from_dentry(v9ses, dentry, &wnames);
+        if (n < 0) {
+                fid = ERR_PTR(n);
+                goto err_out;
+        }
         clone = 1;
         i = 0;
         while (i < n) {
                 l = min(n - i, P9_MAXWELEM);
+                /*
+                 * We need to hold rename lock when doing a multipath
+                 * walk to ensure none of the patch component change
+                 */
                 fid = p9_client_walk(fid, l, &wnames[i], clone);
                 if (IS_ERR(fid)) {
                         if (old_fid) {
@@ -194,15 +234,17 @@ struct p9_fid *v9fs_fid_lookup(struct dentry *dentry)
                                 p9_client_clunk(old_fid);
                         }
                         kfree(wnames);
-                        return fid;
+                        goto err_out;
                 }
                 old_fid = fid;
                 i += l;
                 clone = 0;
         }
-
         kfree(wnames);
+fid_out:
         v9fs_fid_add(dentry, fid);
+err_out:
+        up_read(&v9ses->rename_sem);
         return fid;
 }