overlayfs: barriers for opening upper-layer directory

make sure that
	a) all stores done by opening struct file don't leak past storing
the reference in od->upperfile
	b) the lockless side has read dependency barrier

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

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 910553f37aca..8c8ce9d87ba3 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -454,12 +454,13 @@ static int ovl_dir_fsync(struct file *file, loff_t start, loff_t end,
         if (!od->is_upper && ovl_path_type(dentry) == OVL_PATH_MERGE) {
                 struct inode *inode = file_inode(file);
 
-                realfile = od->upperfile;
+                realfile =lockless_dereference(od->upperfile);
                 if (!realfile) {
                         struct path upperpath;
 
                         ovl_path_upper(dentry, &upperpath);
                         realfile = ovl_path_open(&upperpath, O_RDONLY);
+                        smp_mb__before_spinlock();
                         mutex_lock(&inode->i_mutex);
                         if (!od->upperfile) {
                                 if (IS_ERR(realfile)) {