dcache: Reduce the scope of i_lock in d_splice_alias

i_lock is only needed until __d_find_any_alias calls dget on the alias
dentry.  After that the reference to new ensures that dentry_kill and
d_delete will not remove the inode from the dentry, and remove the
dentry from the inode->d_entry list.

The inode i_lock came to be held over the the __d_move calls in
d_splice_alias through a series of introduction of locks with
increasing smaller scope.  First it was the dcache_lock, then
it was the dcache_inode_lock, and finally inode->i_lock.

Furthermore inode->i_lock is not held over any other calls
to d_move or __d_move so it can not provide any meaningful
rename protection.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 3 insertions, 4 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index e3b44ca75a1b..5c33aeb0f68f 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2718,7 +2718,7 @@ struct dentry *d_ancestor(struct dentry *p1, struct dentry *p2)
  * This helper attempts to cope with remotely renamed directories
  *
  * It assumes that the caller is already holding
- * dentry->d_parent->d_inode->i_mutex, inode->i_lock and rename_lock
+ * dentry->d_parent->d_inode->i_mutex, and rename_lock
  *
  * Note: If ever the locking in lock_rename() changes, then please
  * remember to update this too...
@@ -2744,7 +2744,6 @@ out_unalias:
         __d_move(alias, dentry, false);
         ret = 0;
 out_err:
-        spin_unlock(&inode->i_lock);
         if (m2)
                 mutex_unlock(m2);
         if (m1)
@@ -2790,10 +2789,11 @@ struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
         if (S_ISDIR(inode->i_mode)) {
                 struct dentry *new = __d_find_any_alias(inode);
                 if (unlikely(new)) {
+                        /* The reference to new ensures it remains an alias */
+                        spin_unlock(&inode->i_lock);
                         write_seqlock(&rename_lock);
                         if (unlikely(d_ancestor(new, dentry))) {
                                 write_sequnlock(&rename_lock);
-                                spin_unlock(&inode->i_lock);
                                 dput(new);
                                 new = ERR_PTR(-ELOOP);
                                 pr_warn_ratelimited(
@@ -2812,7 +2812,6 @@ struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
                         } else {
                                 __d_move(new, dentry, false);
                                 write_sequnlock(&rename_lock);
-                                spin_unlock(&inode->i_lock);
                                 security_d_instantiate(new, inode);
                         }
                         iput(inode);