ovl: do not open/llseek lower file with upper sb_writers held

overlayfs file open (ovl_maybe_lookup_lowerdata) and overlay file llseek
take the ovl_inode_lock, without holding upper sb_writers.

In case of nested lower overlay that uses same upper fs as this overlay,
lockdep will warn about (possibly false positive) circular lock
dependency when doing open/llseek of lower ovl file during copy up with
our upper sb_writers held, because the locking ordering seems reverse to
the locking order in ovl_copy_up_start():

- lower ovl_inode_lock
- upper sb_writers

Let the copy up "transaction" keeps an elevated mnt write count on upper
mnt, but leaves taking upper sb_writers to lower level helpers only when
they actually need it.  This allows to avoid holding upper sb_writers
during lower file open/llseek and prevents the lockdep warning.

Minimizing the scope of upper sb_writers during copy up is also needed
for fixing another possible deadlocks by a following patch.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index af11c83b7a25..12156e2091ad 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -670,6 +670,10 @@ bool ovl_already_copied_up(struct dentry *dentry, int flags)
 	return false;
 }
 
+/*
+ * The copy up "transaction" keeps an elevated mnt write count on upper mnt,
+ * but leaves taking freeze protection on upper sb to lower level helpers.
+ */
 int ovl_copy_up_start(struct dentry *dentry, int flags)
 {
 	struct inode *inode = d_inode(dentry);
@@ -682,7 +686,7 @@ int ovl_copy_up_start(struct dentry *dentry, int flags)
 	if (ovl_already_copied_up_locked(dentry, flags))
 		err = 1; /* Already copied up */
 	else
-		err = ovl_want_write(dentry);
+		err = ovl_get_write_access(dentry);
 	if (err)
 		goto out_unlock;
 
@@ -695,7 +699,7 @@ out_unlock:
 
 void ovl_copy_up_end(struct dentry *dentry)
 {
-	ovl_drop_write(dentry);
+	ovl_put_write_access(dentry);
 	ovl_inode_unlock(d_inode(dentry));
 }