[PATCH 16/53] ovl: drop dir lock for lookups in impure readdir

[NeilBrown <neilb@ownmail.net>]

From: NeilBrown <neil@brown.name>

When performing an "impure" readdir, ovl needs to perform a lookup on some
of the names that it found.
With proposed locking changes it will not be possible to perform this
lookup (in particular, not safe to wait for d_alloc_parallel()) while
holding a lock on the directory.

ovl doesn't really need the lock at this point.  It has already iterated
the directory and has cached a list of the contents.  It now needs to
gather extra information about some contents.  It can do this without
the lock.

After gathering that info it needs to retake the lock for API
correctness.  After doing this it must check IS_DEADDIR() again to
ensure readdir always returns -ENOENT on a removed directory.

Note that while ->iterate_shared is called with a shared lock, ovl uses
WRAP_DIR_ITER() so an exclusive lock is held and so we drop and retake
that exclusive lock.

As the directory is no longer locked in ovl_cache_update() we need
dget_parent() to get a reference to the parent.

Signed-off-by: NeilBrown <neil@brown.name>
---
 fs/overlayfs/readdir.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 1dcc75b3a90f..d5123b37921c 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -568,13 +568,12 @@ static int ovl_cache_update(const struct path *path, struct ovl_cache_entry *p,
 			goto get;
 		}
 		if (p->len == 2) {
-			/* we shall not be moved */
-			this = dget(dir->d_parent);
+			this = dget_parent(dir);
 			goto get;
 		}
 	}
 	/* This checks also for xwhiteouts */
-	this = lookup_one(mnt_idmap(path->mnt), &QSTR_LEN(p->name, p->len), dir);
+	this = lookup_one_unlocked(mnt_idmap(path->mnt), &QSTR_LEN(p->name, p->len), dir);
 	if (IS_ERR_OR_NULL(this) || !this->d_inode) {
 		/* Mark a stale entry */
 		p->is_whiteout = true;
@@ -666,11 +665,12 @@ static int ovl_dir_read_impure(const struct path *path,  struct list_head *list,
 	if (err)
 		return err;
 
+	inode_unlock(path->dentry->d_inode);
 	list_for_each_entry_safe(p, n, list, l_node) {
 		if (!name_is_dot_dotdot(p->name, p->len)) {
 			err = ovl_cache_update(path, p, true);
 			if (err)
-				return err;
+				break;
 		}
 		if (p->ino == p->real_ino) {
 			list_del(&p->l_node);
@@ -680,14 +680,19 @@ static int ovl_dir_read_impure(const struct path *path,  struct list_head *list,
 			struct rb_node *parent = NULL;
 
 			if (WARN_ON(ovl_cache_entry_find_link(p->name, p->len,
-							      &newp, &parent)))
-				return -EIO;
+							      &newp, &parent))) {
+				err = -EIO;
+				break;
+			}
 
 			rb_link_node(&p->node, parent, newp);
 			rb_insert_color(&p->node, root);
 		}
 	}
-	return 0;
+	inode_lock(path->dentry->d_inode);
+	if (IS_DEADDIR(path->dentry->d_inode))
+		err = -ENOENT;
+	return err;
 }
 
 static struct ovl_dir_cache *ovl_cache_get_impure(const struct path *path)
-- 
2.50.0.107.gf914562f5916.dirty