linux-kernel - [PATCH 2/4] fs/inode.c: do not take i_lock in __(insert|remove)_inode

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Message-Id: <1348219866-1799-3-git-send-email-yan@linux.vnet.ibm.com>
Date:	Fri, 21 Sep 2012 17:31:04 +0800
From:	Guo Chao <yan@...ux.vnet.ibm.com>
To:	viro@...iv.linux.org.uk
Cc:	dchinner@...hat.com, hch@...radead.org, jack@...e.cz,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 2/4] fs/inode.c: do not take i_lock in __(insert|remove)_inode_hash

At the beginning of fs/inode.c, the comment suggests both inode_hash_lock and
i_lock protect i_hash. I wonder why two locks are needed.

Grep the source code shows that i_hash is accessed in limited and
well-defined places:

        1. In inode_unhashed()
        2. In some special cases, as argument to hlist_add_fake()
        3. Traverse inode hash list
        4. Add/remove inode to/from inode hash list.

Case #1, #2 are not in any locking context. Case #3, #4 will hold
inode_hash_lock. i_hash is not accessed by other ways. This suggests that
inode_hash_lock alone protects i_hash well.

In __(insert|remove)_inode_hash(), our only goal is to manipulate i_hash, do
not take i_lock in these functions.

Signed-off-by: Guo Chao <yan@...ux.vnet.ibm.com>
---
 fs/inode.c |    4 ----
 1 file changed, 4 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index d2d15aa..54e4b29 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -463,9 +463,7 @@ void __insert_inode_hash(struct inode *inode, unsigned long hashval)
 	struct hlist_head *b = inode_hashtable + hash(inode->i_sb, hashval);
 
 	spin_lock(&inode_hash_lock);
-	spin_lock(&inode->i_lock);
 	hlist_add_head(&inode->i_hash, b);
-	spin_unlock(&inode->i_lock);
 	spin_unlock(&inode_hash_lock);
 }
 EXPORT_SYMBOL(__insert_inode_hash);
@@ -479,9 +477,7 @@ EXPORT_SYMBOL(__insert_inode_hash);
 void __remove_inode_hash(struct inode *inode)
 {
 	spin_lock(&inode_hash_lock);
-	spin_lock(&inode->i_lock);
 	hlist_del_init(&inode->i_hash);
-	spin_unlock(&inode->i_lock);
 	spin_unlock(&inode_hash_lock);
 }
 EXPORT_SYMBOL(__remove_inode_hash);
-- 
1.7.9.5

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