linux-kernel - [PATCH] Allow d_splice_alias to accept hashed dentries

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Date:	Thu,  2 Jun 2016 19:59:37 -0400
From:	green@...uxhacker.ru
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	"J. Bruce Fields" <bfields@...hat.com>,
	"<linux-kernel@...r.kernel.org> Mailing List" 
	<linux-kernel@...r.kernel.org>,
	"<linux-fsdevel@...r.kernel.org>" <linux-fsdevel@...r.kernel.org>,
	Oleg Drokin <green@...uxhacker.ru>
Subject: [PATCH] Allow d_splice_alias to accept hashed dentries

From: Oleg Drokin <green@...uxhacker.ru>

The comment at the top of the function already permits it,
just take out the assetrtion and update __d_add to
be aware of this. Move the BUG_ON to d_add which is the
other user of __d_add and presumably we do not want to
allow hashed dentries as input there.

Signed-off-by: Oleg Drokin <green@...uxhacker.ru>
---
So seeing how there's only one other __d_add user, I just moved the BUG_ON
to d_add and allowed __d_add to not call _d_rehash if the dentry is
already hashed.
This seems to be working as expected and I don't see any problems with it.
Of course I am probably missing some other important consideration ;)

 fs/dcache.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index ad4a542..0e35049 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2565,7 +2565,8 @@ static inline void __d_add(struct dentry *dentry, struct inode *inode)
 		raw_write_seqcount_end(&dentry->d_seq);
 		__fsnotify_d_instantiate(dentry);
 	}
-	_d_rehash(dentry);
+	if (d_unhashed(dentry))
+		_d_rehash(dentry);
 	if (dir)
 		end_dir_add(dir, n);
 	spin_unlock(&dentry->d_lock);
@@ -2584,6 +2585,8 @@ static inline void __d_add(struct dentry *dentry, struct inode *inode)
 
 void d_add(struct dentry *entry, struct inode *inode)
 {
+	BUG_ON(!d_unhashed(entry));
+
 	if (inode) {
 		security_d_instantiate(entry, inode);
 		spin_lock(&inode->i_lock);
@@ -2986,8 +2989,6 @@ struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
 	if (IS_ERR(inode))
 		return ERR_CAST(inode);
 
-	BUG_ON(!d_unhashed(dentry));
-
 	if (!inode)
 		goto out;
 
-- 
2.1.0