linux-kernel - [RFC PATCH v2 1/2] fscrypt: remove d_lock on reading DCACHE_ENCRYPTED_WITH

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Date:   Tue, 6 Nov 2018 20:20:51 +0800
From:   Gao Xiang <gaoxiang25@...wei.com>
To:     "Theodore Y . Ts'o" <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Eric Biggers <ebiggers@...gle.com>
CC:     <linux-fscrypt@...r.kernel.org>, <linux-fsdevel@...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 1/2] fscrypt: remove d_lock on reading DCACHE_ENCRYPTED_WITH_KEY

It seems there is no need to take d_lock when accessing
dentry->d_flags & DCACHE_ENCRYPTED_WITH_KEY in
fscrypt_d_revalidate. It only needs to be serialized for updating.

Signed-off-by: Gao Xiang <gaoxiang25@...wei.com>
---

Change log v2:
 - Fix negative dentry (NULL d_inode) issue as Eric pointed out;
 - Fix to use '__lockref_is_dead()' as Eric pointed out before;

[original message]
Hi,
 At glance, I have no idea why fscrypt_d_revalidate disables
RCU-lookup. Therefore I made patches to raise a question and
do some test (eg. fsstress w/ and w/o drop_caches) and it
seems to work.

Thanks,
Gao Xiang

 fs/crypto/crypto.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/crypto/crypto.c b/fs/crypto/crypto.c
index 0f46cf550907..b38c574f70ac 100644
--- a/fs/crypto/crypto.c
+++ b/fs/crypto/crypto.c
@@ -329,9 +329,8 @@ static int fscrypt_d_revalidate(struct dentry *dentry, unsigned int flags)
 		return 0;
 	}
 
-	spin_lock(&dentry->d_lock);
-	cached_with_key = dentry->d_flags & DCACHE_ENCRYPTED_WITH_KEY;
-	spin_unlock(&dentry->d_lock);
+	cached_with_key = READ_ONCE(dentry->d_flags) &
+		DCACHE_ENCRYPTED_WITH_KEY;
 	dir_has_key = (d_inode(dir)->i_crypt_info != NULL);
 	dput(dir);
 
-- 
2.14.4