[PATCH 12/19] VFS: enhance d_splice_alias to accommodate shared-lock updates

[NeilBrown <neilb@suse.de>]

d_splice_alias() - via __d_unalias() - currently assumes that taking a
shared lock on the parent directory locks against any change to the
parent/name of the dentry.  This will no longer be the case with
shared-lock updates.  We also need a DCACHE_PAR_UPDATE lock on the
dentry.

This patch adds a call to d_update_trylock() to get this lock -if
possible.  This lock ensures that the test on ->d_parent and ->d_name in
d_update_lock() will not be invalidated by the __d_move() in __d_unalias.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/dcache.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index e705696ca57e..fb331596f1b1 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -3036,13 +3036,17 @@ static int __d_unalias(struct dentry *dentry, struct dentry *alias)
 		goto out_err;
 	m2 = &alias->d_parent->d_inode->i_rwsem;
 out_unalias:
+	if (!d_update_trylock(dentry, NULL, NULL))
+		goto out_err;
 	if (alias->d_op && alias->d_op->d_unalias_trylock &&
 	    !alias->d_op->d_unalias_trylock(alias))
-		goto out_err;
+		goto out_err2;
 	__d_move(alias, dentry, false);
 	if (alias->d_op && alias->d_op->d_unalias_unlock)
 		alias->d_op->d_unalias_unlock(alias);
 	ret = 0;
+out_err2:
+	d_update_unlock(dentry);
 out_err:
 	if (m2)
 		up_read(m2);
@@ -3073,6 +3077,10 @@ static int __d_unalias(struct dentry *dentry, struct dentry *alias)
  * In that case, we know that the inode will be a regular file, and also this
  * will only occur during atomic_open. So we need to check for the dentry
  * being already hashed only in the final case.
+ *
+ * @dentry must have a valid ->d_parent and that directory must be
+ * locked (i_rwsem) either exclusively or shared.  If shared then
+ * @dentry must have %DCACHE_PAR_LOOKUP or %DCACHE_PAR_UPDATE set.
  */
 struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
 {
-- 
2.47.1