non-RCU analogue of the previous commit

new helper: choose_mountpoint().  Wrapper around choose_mountpoint_rcu(),
similar to lookup_mnt() vs. __lookup_mnt().  follow_dotdot() switched to
it.  Now we don't grab mount_lock exclusive anymore; note that the
primitive used non-RCU mount traversals in other direction (lookup_mnt())
doesn't bother with that either - it uses mount_lock seqcount instead.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namei.c b/fs/namei.c
index 6529c25..669a0df 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -605,10 +605,9 @@ static void terminate_walk(struct nameidata *nd)
 }
 
 /* path_put is needed afterwards regardless of success or failure */
-static bool legitimize_path(struct nameidata *nd,
-                           struct path *path, unsigned seq)
+static bool __legitimize_path(struct path *path, unsigned seq, unsigned mseq)
 {
-       int res = __legitimize_mnt(path->mnt, nd->m_seq);
+       int res = __legitimize_mnt(path->mnt, mseq);
        if (unlikely(res)) {
                if (res > 0)
                        path->mnt = NULL;
@@ -622,6 +621,12 @@ static bool legitimize_path(struct nameidata *nd,
        return !read_seqcount_retry(&path->dentry->d_seq, seq);
 }
 
+static inline bool legitimize_path(struct nameidata *nd,
+                           struct path *path, unsigned seq)
+{
+       return __legitimize_path(path, nd->m_seq, seq);
+}
+
 static bool legitimize_links(struct nameidata *nd)
 {
        int i;
@@ -1154,6 +1159,31 @@ static bool choose_mountpoint_rcu(struct mount *m, const struct path *root,
        return false;
 }
 
+static bool choose_mountpoint(struct mount *m, const struct path *root,
+                             struct path *path)
+{
+       bool found;
+
+       rcu_read_lock();
+       while (1) {
+               unsigned seq, mseq = read_seqbegin(&mount_lock);
+
+               found = choose_mountpoint_rcu(m, root, path, &seq);
+               if (unlikely(!found)) {
+                       if (!read_seqretry(&mount_lock, mseq))
+                               break;
+               } else {
+                       if (likely(__legitimize_path(path, seq, mseq)))
+                               break;
+                       rcu_read_unlock();
+                       path_put(path);
+                       rcu_read_lock();
+               }
+       }
+       rcu_read_unlock();
+       return found;
+}
+
 /*
  * Perform an automount
  * - return -EISDIR to tell follow_managed() to stop and return the path we
@@ -1756,22 +1786,14 @@ static struct dentry *follow_dotdot(struct nameidata *nd,
        if (path_equal(&nd->path, &nd->root))
                goto in_root;
        if (unlikely(nd->path.dentry == nd->path.mnt->mnt_root)) {
-               struct path path = nd->path;
-               path_get(&path);
-               while (1) {
-                       if (!follow_up(&path)) {
-                               path_put(&path);
-                               goto in_root;
-                       }
-                       if (path_equal(&path, &nd->root)) {
-                               path_put(&path);
-                               goto in_root;
-                       }
-                       if (path.dentry != nd->path.mnt->mnt_root)
-                               break;
-               }
+               struct path path;
+
+               if (!choose_mountpoint(real_mount(nd->path.mnt),
+                                      &nd->root, &path))
+                       goto in_root;
                path_put(&nd->path);
                nd->path = path;
+               nd->inode = path.dentry->d_inode;
                if (unlikely(nd->flags & LOOKUP_NO_XDEV))
                        return ERR_PTR(-EXDEV);
        }