linux-kernel - [PATCH 1/4] fs/inode.c: do not take i

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Date:	Fri, 21 Sep 2012 17:31:03 +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 1/4] fs/inode.c: do not take i_lock on newly allocated inode

Does current thread have the exclusive access to newly allocated inode?
The chanllege comes from individual filesystem's ->alloc_inode method.
It may exposure the inode to private lists, thus give other threads a chance
to access the inode.

That should not happen, however, because the inode is not even gone through a
basic initialization. The locks may not be initialized yet, the inode is not
ready for parallel access.

Thus, from getting an inode from alloc_inode(), till we put it on a list and
that list becomes accessible, we can safely manipulate the inode without
taking i_lock.

This patch fixes three places that take the unnecessary i_lock.

Note in iget5_locked(), I have no strong proof set() should not put the inode
into private lists. Any suggestion?

Signed-off-by: Guo Chao <yan@...ux.vnet.ibm.com>
---
 fs/inode.c |    6 ------
 1 file changed, 6 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index ac8d904..d2d15aa 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -895,9 +895,7 @@ struct inode *new_inode_pseudo(struct super_block *sb)
 	struct inode *inode = alloc_inode(sb);
 
 	if (inode) {
-		spin_lock(&inode->i_lock);
 		inode->i_state = 0;
-		spin_unlock(&inode->i_lock);
 		INIT_LIST_HEAD(&inode->i_sb_list);
 	}
 	return inode;
@@ -1015,10 +1013,8 @@ struct inode *iget5_locked(struct super_block *sb, unsigned long hashval,
 			if (set(inode, data))
 				goto set_failed;
 
-			spin_lock(&inode->i_lock);
 			inode->i_state = I_NEW;
 			hlist_add_head(&inode->i_hash, head);
-			spin_unlock(&inode->i_lock);
 			inode_sb_list_add(inode);
 			spin_unlock(&inode_hash_lock);
 
@@ -1082,10 +1078,8 @@ struct inode *iget_locked(struct super_block *sb, unsigned long ino)
 		old = find_inode_fast(sb, head, ino);
 		if (!old) {
 			inode->i_ino = ino;
-			spin_lock(&inode->i_lock);
 			inode->i_state = I_NEW;
 			hlist_add_head(&inode->i_hash, head);
-			spin_unlock(&inode->i_lock);
 			inode_sb_list_add(inode);
 			spin_unlock(&inode_hash_lock);
 
-- 
1.7.9.5

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