Re: [PATCH] VFS bugfix: two read_inode() calles without clear_inode() call between

["Artem B. Bityuckiy" <dedekind@infradead.org>]

Hello Andrew,

here you can find a new patch for the VFS bug which was discussed at
http://lkml.org/lkml/2005/4/27/84

I added wake_up_inode() invocation just as Miklos suggested.


Bug symptoms
~~~~~~~~~~~~
For the same inode VFS calls read_inode() twice and doesn't call
clear_inode() between the two read_inode() invocations.

Bug description
~~~~~~~~~~~~~~~
Suppose we have an inode which has zero reference count but is still in
the inode cache. Suppose kswapd invokes shrink_icache_memory() to free
some RAM. In prune_icache() inodes are removed from i_hash. prune_icache
() is then going to call clear_inode(), but drops the inode_lock
spinlock before this. If in this moment another task calls iget() for an
inode which was just removed from i_hash by prune_icache(), then iget()
invokes read_inode() for this inode, because it is *already removed*
from i_hash.

The end result is: we call iget(#N) then iput(#N); inode #N has zero
i_count now and is in the inode cache; kswapd starts. kswapd removes the
inode #N from i_hash ans is preempted; we call iget(#N) again;
read_inode() is invoked as the result; but we expect clear_inode()
before.

Fix
~~~~~~~
To fix the bug I remove inodes from i_hash later, when clear_inode() is
actually called. I remove them from i_hash under spinlock protection.
Since the i_state is set to I_FREEING, it is safe to do this. The others
will sleep waiting for the inode state change.

I also postpone removing inodes from i_sb_list. It is not compulsory to
do so but I do it for readability reasons. Inodes are added/removed to
the lists together everywhere in the code and there is no point to
change this rule. This is harmless because the only user of i_sb_list
which somehow may interfere with me (invalidate_list()) is excluded by
the iprune_sem mutex.

The same race is possible in invalidate_list() so I do the same for it.

The patch is against linux 2.6.11.5.
The patch was tested for JFFS2.

Please. apply/comment.

Cheers,
Artem.

-- 
Best Regards,
Artem B. Bityuckiy,
St.-Petersburg, Russia.

diff -auNrp linux-2.6.11.5/fs/inode.c linux-2.6.11.5_fixed/fs/inode.c
--- linux-2.6.11.5/fs/inode.c	2005-03-19 09:35:04.000000000 +0300
+++ linux-2.6.11.5_fixed/fs/inode.c	2005-05-04 14:51:14.000000000 +0400
@@ -284,6 +284,13 @@ static void dispose_list(struct list_hea
 		if (inode->i_data.nrpages)
 			truncate_inode_pages(&inode->i_data, 0);
 		clear_inode(inode);
+		
+		spin_lock(&inode_lock);
+		hlist_del_init(&inode->i_hash);
+		list_del_init(&inode->i_sb_list);
+		spin_unlock(&inode_lock);
+		
+		wake_up_inode(inode);
 		destroy_inode(inode);
 		nr_disposed++;
 	}
@@ -319,8 +326,6 @@ static int invalidate_list(struct list_h
 		inode = list_entry(tmp, struct inode, i_sb_list);
 		invalidate_inode_buffers(inode);
 		if (!atomic_read(&inode->i_count)) {
-			hlist_del_init(&inode->i_hash);
-			list_del(&inode->i_sb_list);
 			list_move(&inode->i_list, dispose);
 			inode->i_state |= I_FREEING;
 			count++;
@@ -455,8 +460,6 @@ static void prune_icache(int nr_to_scan)
 			if (!can_unuse(inode))
 				continue;
 		}
-		hlist_del_init(&inode->i_hash);
-		list_del_init(&inode->i_sb_list);
 		list_move(&inode->i_list, &freeable);
 		inode->i_state |= I_FREEING;
 		nr_pruned++;