[PATCH 7/7] VFS: introduce dentry_lock_in()

[NeilBrown <neil@brown.name>]

A few callers operate on a dentry which they already have - unlike the
normal case where a lookup proceeds an operation.

For these callers dentry_lock_in() is provided where other callers would
use dentry_lookup().  The call will fail if, after the lock was
gained, the child is no longer a child of the given parent.

When the operation completes done_dentry_lookup() must be called.  An
extra reference is taken when the dentry_lock_in() call succeeds
and will be dropped by done_dentry_lookup().

This will be used in smb/server, ecryptfs, and overlayfs, each of which
have their own lock_parent() or parent_lock() or similar; and a few
other places which lock the parent but don't check if the parent is
still correct (often because rename isn't supported so parent cannot be
incorrect).

Signed-off-by: NeilBrown <neil@brown.name>
---
 fs/namei.c            | 26 ++++++++++++++++++++++++++
 include/linux/namei.h |  1 +
 2 files changed, 27 insertions(+)

diff --git a/fs/namei.c b/fs/namei.c
index ae8079916ac6..ed656a1e458c 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1836,6 +1836,32 @@ struct dentry *dentry_lookup_killable(struct mnt_idmap *idmap,
 }
 EXPORT_SYMBOL(dentry_lookup_killable);
 
+/**
+ * dentry_lock_in: lock a dentry in given parent prior to dir ops
+ * @child: the dentry to lock
+ * @parent: the dentry of the assumed parent
+ *
+ * The child is locked - currently by taking i_rwsem on the parent - to
+ * prepare for create/remove operations.  If the given parent is no longer
+ * the parent of the dentry after the lock is gained, the lock is released
+ * and the call fails (returns %false).
+ *
+ * A reference is taken to the child on success.  The lock and reference
+ * must both be dropped by done_dentry_lookup() after the operation completes.
+ */
+bool dentry_lock_in(struct dentry *child, struct dentry *parent)
+{
+	inode_lock_nested(d_inode(parent), I_MUTEX_PARENT);
+	if (child->d_parent == parent) {
+		/* get the child to balance with done_dentry_lookup() which puts it. */
+		dget(child);
+		return true;
+	}
+	inode_unlock(d_inode(parent));
+	return false;
+}
+EXPORT_SYMBOL(dentry_lock_in);
+
 /**
  * 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 c86d9683563c..61ab251237e4 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -104,6 +104,7 @@ int rename_lookup(struct renamedata *rd, int lookup_flags);
 int rename_lookup_noperm(struct renamedata *rd, int lookup_flags);
 int rename_lookup_hashed(struct renamedata *rd, int lookup_flags);
 void done_rename_lookup(struct renamedata *rd);
+bool dentry_lock_in(struct dentry *child, struct dentry *parent);
 
 /**
  * mode_strip_umask - handle vfs umask stripping
-- 
2.49.0