[PATCH 7/7] ovl: stop using lookup_one() in ovl_iterate().

[NeilBrown]

From: NeilBrown <neil at brown.name>

lookup_one() is expected to be removed as it does not fit well with
proposed changes to directory locking.
Specifically d_alloc_parallel() will be ordered outside of i_rwsem
and as iterate_shared() is called with i_rwsem held it is not safe
to call d_alloc_parallel().

We can instead call d_alloc_trylock() and then call the ->lookup, but
that can fail if there is a lookup attempt concurrent with the
readdir().

ovl cannot afford for the lookup to fail as that could produce incorrect
results, and it cannot safely drop i_rwsem temporarily as that could
introduce races with handling of the directory cache.

Instead we rely on the fact that ovl_iterate() has an exclusive lock on
the directory, so any concurrent lookup will wait for the ovl_iterate()
call to complete.  We allocate a separate dentry and if the lookup is
successful, it is hashed with the result.

When the concurrent lookup gets i_rwsem it mustn't do its own lookup -
it must use the existing dentry.  This is found, if it exists, using
try_lookup_noperm().

Signed-off-by: NeilBrown <neil at brown.name>
---
 fs/overlayfs/namei.c   | 12 ++++++++++++
 fs/overlayfs/readdir.c | 26 ++++++++++++++++++++++++--
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index ca899fdfaafd..f56e33fd4d6b 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -1385,6 +1385,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 	struct ovl_fs *ofs = OVL_FS(dentry->d_sb);
 	struct ovl_entry *poe = OVL_E(dentry->d_parent);
 	bool check_redirect = (ovl_redirect_follow(ofs) || ofs->numdatalayer);
+	struct dentry *alias;
 	int err;
 	struct ovl_lookup_ctx ctx = {
 		.dentry = dentry,
@@ -1399,6 +1400,17 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 	if (dentry->d_name.len > ofs->namelen)
 		return ERR_PTR(-ENAMETOOLONG);
 
+	/*
+	 * The existence of this in-lookup dentry might have forced
+	 * readdir to do the lookup with a new dentry.  If so we must
+	 * return that one.
+	 */
+	alias = try_lookup_noperm(&QSTR_LEN(dentry->d_name.name,
+					    dentry->d_name.len),
+				  dentry->d_parent);
+	if (alias && !IS_ERR(alias))
+		return alias;
+
 	with_ovl_creds(dentry->d_sb)
 		err = ovl_lookup_layers(&ctx, &d);
 
diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index e7fe29cb6028..bc41da4bace8 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -574,8 +574,30 @@ static int ovl_cache_update(const struct path *path, struct ovl_cache_entry *p,
 		}
 	}
 	/* This checks also for xwhiteouts */
-	this = lookup_one(mnt_idmap(path->mnt), &QSTR_LEN(p->name, p->len), dir);
-	if (IS_ERR_OR_NULL(this) || !this->d_inode) {
+	this = d_alloc_trylock(dir, &QSTR_LEN(p->name, p->len));
+	if (this == ERR_PTR(-EWOULDBLOCK)) {
+		/*
+		 * Some other thread is looking up this name and will
+		 * block on i_rwsem before it can complete the lookup.
+		 * We will do the lookup in a new dentry and when that
+		 * lookup gets a turn it will find and return this
+		 * dentry.
+		 */
+		this = d_alloc_name(dir, p->name);
+		if (!this)
+			this = ERR_PTR(-ENOMEM);
+	}
+	if (!IS_ERR(this) && d_unhashed(this)) {
+		/* Either we got an in-lookup or we made our own unhashed */
+		struct dentry *alias = ovl_lookup(dir->d_inode, this, 0);
+
+		d_lookup_done(this);
+		if (alias) {
+			dput(this);
+			this = alias;
+		}
+	}
+	if (IS_ERR(this) || !this->d_inode) {
 		/* Mark a stale entry */
 		p->is_whiteout = true;
 		if (IS_ERR(this)) {
-- 
2.50.0.107.gf914562f5916.dirty