From: NeilBrown <neil@brown.name>

Some ->lookup handlers will need to drop and retake the parent lock, so
they can safely use d_alloc_parallel().

->lookup can be called with the parent lock either exclusive or shared.

A new flag, LOOKUP_SHARED, tells ->lookup how the parent is locked.

This is rather ugly, but will be gone soon after we move
d_alloc_parallel() out of the directory lock as ->lookup() will *always*
called with a shared lock on the parent.

Signed-off-by: NeilBrown <neil@brown.name>
---
 fs/namei.c            | 20 +++++++++++---------
 include/linux/namei.h |  3 ++-
 2 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 8d8d2af185f1..e978a75eb8a1 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1933,7 +1933,7 @@ static noinline struct dentry *lookup_slow(const struct qstr *name,
 	struct inode *inode = dir->d_inode;
 	struct dentry *res;
 	inode_lock_shared(inode);
-	res = __lookup_slow(name, dir, flags);
+	res = __lookup_slow(name, dir, flags | LOOKUP_SHARED);
 	inode_unlock_shared(inode);
 	return res;
 }
@@ -1947,7 +1947,7 @@ static struct dentry *lookup_slow_killable(const struct qstr *name,
 
 	if (inode_lock_shared_killable(inode))
 		return ERR_PTR(-EINTR);
-	res = __lookup_slow(name, dir, flags);
+	res = __lookup_slow(name, dir, flags | LOOKUP_SHARED);
 	inode_unlock_shared(inode);
 	return res;
 }
@@ -4440,12 +4440,14 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
 	int error, create_error;
 	umode_t mode;
 	bool got_write;
+	unsigned int shared_flag;
 
 retry:
 	open_flag = op->open_flag;
 	got_write = false;
 	mode = op->mode;
 	create_error = 0;
+	shared_flag = (open_flag & O_CREAT) ? 0 : LOOKUP_SHARED;
 
 	if (open_flag & (O_CREAT | O_TRUNC | O_WRONLY | O_RDWR)) {
 		got_write = !mnt_want_write(nd->path.mnt);
@@ -4454,10 +4456,10 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
 		 * a different error; we'll be dropping this one anyway.
 		 */
 	}
-	if (open_flag & O_CREAT)
-		inode_lock(dir_inode);
-	else
+	if (shared_flag)
 		inode_lock_shared(dir_inode);
+	else
+		inode_lock(dir_inode);
 
 	if (unlikely(IS_DEADDIR(dir_inode))) {
 		dentry = ERR_PTR(-ENOENT);
@@ -4526,7 +4528,7 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
 
 	if (d_in_lookup(dentry)) {
 		struct dentry *res = dir_inode->i_op->lookup(dir_inode, dentry,
-							     nd->flags);
+							     nd->flags | shared_flag);
 		d_lookup_done(dentry);
 		if (unlikely(res)) {
 			if (IS_ERR(res)) {
@@ -4574,10 +4576,10 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
 		if (file->f_mode & FMODE_OPENED)
 			fsnotify_open(file);
 	}
-	if ((open_flag & O_CREAT) || create_error)
-		inode_unlock(dir_inode);
-	else
+	if (shared_flag)
 		inode_unlock_shared(dir_inode);
+	else
+		inode_unlock(dir_inode);
 
 	if (got_write)
 		mnt_drop_write(nd->path.mnt);
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 86d657b24fc6..65f0f5712885 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -32,8 +32,9 @@ enum { MAX_NESTED_LINKS = 8 };
 #define LOOKUP_CREATE		BIT(17)	/* ... in object creation */
 #define LOOKUP_EXCL		BIT(18)	/* ... in target must not exist */
 #define LOOKUP_RENAME_TARGET	BIT(19)	/* ... in destination of rename() */
+#define LOOKUP_SHARED		BIT(20) /* Parent lock is held shared */
 
-/* 4 spare bits for intent */
+/* 3 spare bits for intent */
 
 /* Scoping flags for lookup. */
 #define LOOKUP_NO_SYMLINKS	BIT(24) /* No symlink crossing. */
-- 
2.50.0.107.gf914562f5916.dirty