[PATCH 5/7] VFS: add dentry_lookup_killable()

[NeilBrown <neil@brown.name>]

btrfs/ioctl.c uses a "killable" lock on the directory when creating an
destroying subvols.  overlayfs also does this.

This patch adds dentry_lookup_killable() for these users.

Possibly all dentry_lookup should be killable as there is no down-side,
but that can come in a later patch.

Signed-off-by: NeilBrown <neil@brown.name>
---
 fs/namei.c            | 37 +++++++++++++++++++++++++++++++++++++
 include/linux/namei.h |  3 +++
 2 files changed, 40 insertions(+)

diff --git a/fs/namei.c b/fs/namei.c
index f292df61565a..46657736c7a0 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1799,6 +1799,43 @@ struct dentry *dentry_lookup(struct mnt_idmap *idmap,
 }
 EXPORT_SYMBOL(dentry_lookup);
 
+/**
+ * dentry_lookup_killable - lookup and lock a name prior to dir ops
+ * @last: the name in the given directory
+ * @base: the directory in which the name is to be found
+ * @lookup_flags: %LOOKUP_xxx flags
+ *
+ * The name is looked up and necessary locks are taken so that
+ * the name can be created or removed.
+ * The "necessary locks" are currently the inode node lock on @base.
+ * If a fatal signal arrives or is already pending the operation is aborted.
+ * The name @last is NOT expected to already have the hash calculated.
+ * Permission checks are performed to ensure %MAY_EXEC access to @base.
+ * Returns: the dentry, suitably locked, or an ERR_PTR().
+ */
+struct dentry *dentry_lookup_killable(struct mnt_idmap *idmap,
+				      struct qstr *last,
+				      struct dentry *base,
+				      unsigned int lookup_flags)
+{
+	struct dentry *dentry;
+	int err;
+
+	err = lookup_one_common(idmap, last, base);
+	if (err < 0)
+		return ERR_PTR(err);
+
+	err = down_write_killable_nested(&base->d_inode->i_rwsem, I_MUTEX_PARENT);
+	if (err)
+		return ERR_PTR(err);
+
+	dentry = lookup_one_qstr_excl(last, base, lookup_flags);
+	if (IS_ERR(dentry))
+		inode_unlock(base->d_inode);
+	return dentry;
+}
+EXPORT_SYMBOL(dentry_lookup_killable);
+
 /**
  * done_dentry_lookup - finish a lookup used for create/delete
  * @dentry:  the target dentry
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 6d2cf4af5f16..8eade32623d8 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -83,6 +83,9 @@ struct dentry *lookup_one_positive_unlocked(struct mnt_idmap *idmap,
 struct dentry *dentry_lookup(struct mnt_idmap *idmap,
 			       struct qstr *last, struct dentry *base,
 			       unsigned int lookup_flags);
+struct dentry *dentry_lookup_killable(struct mnt_idmap *idmap,
+				      struct qstr *last, struct dentry *base,
+				      unsigned int lookup_flags);
 struct dentry *dentry_lookup_noperm(struct qstr *name, struct dentry *base,
 				      unsigned int lookup_flags);
 struct dentry *dentry_lookup_hashed(struct qstr *last, struct dentry *base,
-- 
2.49.0