linux-kernel - [RFC PATCH v2 2/2] fscrypt: enable RCU-walk path for .d

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Date:   Mon, 10 Sep 2018 21:08:57 +0800
From:   Gao Xiang <gaoxiang25@...wei.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>
CC:     Eric Biggers <ebiggers@...gle.com>,
        <linux-fscrypt@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Chao Yu <yuchao0@...wei.com>, Miao Xie <miaoxie@...wei.com>,
        <weidu.du@...wei.com>, Gao Xiang <gaoxiang25@...wei.com>
Subject: [RFC PATCH v2 2/2] fscrypt: enable RCU-walk path for .d_revalidate

This patch attempts to enable RCU-walk for fscrypt.
It looks harmless at glance and could have better
performance than do ref-walk only.

Signed-off-by: Gao Xiang <gaoxiang25@...wei.com>
---
change log v2:
	- READ_ONCE(dir->d_parent) -> READ_ONCE(dentry->d_parent)

 fs/crypto/crypto.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/fs/crypto/crypto.c b/fs/crypto/crypto.c
index b38c574..9bd21c0 100644
--- a/fs/crypto/crypto.c
+++ b/fs/crypto/crypto.c
@@ -319,20 +319,24 @@ static int fscrypt_d_revalidate(struct dentry *dentry, unsigned int flags)
 {
 	struct dentry *dir;
 	int dir_has_key, cached_with_key;
-
-	if (flags & LOOKUP_RCU)
-		return -ECHILD;
-
-	dir = dget_parent(dentry);
-	if (!IS_ENCRYPTED(d_inode(dir))) {
-		dput(dir);
+	struct inode *dir_inode;
+
+	rcu_read_lock();
+repeat:
+	dir = READ_ONCE(dentry->d_parent);
+	dir_inode = d_inode_rcu(dir);
+	if (!IS_ENCRYPTED(dir_inode)) {
+		rcu_read_unlock();
 		return 0;
 	}
+	dir_has_key = (dir_inode->i_crypt_info != NULL);
+	if (unlikely(READ_ONCE(dir->d_lockref.count) < 0 ||
+		READ_ONCE(dentry->d_parent) != dir))
+		goto repeat;
+	rcu_read_unlock();
 
 	cached_with_key = READ_ONCE(dentry->d_flags) &
 		DCACHE_ENCRYPTED_WITH_KEY;
-	dir_has_key = (d_inode(dir)->i_crypt_info != NULL);
-	dput(dir);
 
 	/*
 	 * If the dentry was cached without the key, and it is a
-- 
1.9.1