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 <gaoxian...@huawei.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

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