Commit 4120db47 authored Jul 12, 2005 by Artem B. Bityuckiy Committed by Linus Torvalds Jul 12, 2005

[PATCH] bugfix: two read_inode() calls without clear_inode() call between



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.

Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent 168a9fd6

fs/inode.c

+7 −4

Original line number	Original line	Diff line number	Diff line
	@@ -282,6 +282,13 @@ static void dispose_list(struct list_head *head)
	if (inode->i_data.nrpages)		if (inode->i_data.nrpages)
	truncate_inode_pages(&inode->i_data, 0);		truncate_inode_pages(&inode->i_data, 0);
	clear_inode(inode);		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);		destroy_inode(inode);
	nr_disposed++;		nr_disposed++;
	}		}
	@@ -317,8 +324,6 @@ static int invalidate_list(struct list_head head, struct list_head dispose)
	inode = list_entry(tmp, struct inode, i_sb_list);		inode = list_entry(tmp, struct inode, i_sb_list);
	invalidate_inode_buffers(inode);		invalidate_inode_buffers(inode);
	if (!atomic_read(&inode->i_count)) {		if (!atomic_read(&inode->i_count)) {
	hlist_del_init(&inode->i_hash);
	list_del(&inode->i_sb_list);
	list_move(&inode->i_list, dispose);		list_move(&inode->i_list, dispose);
	inode->i_state \|= I_FREEING;		inode->i_state \|= I_FREEING;
	count++;		count++;
	@@ -439,8 +444,6 @@ static void prune_icache(int nr_to_scan)
	if (!can_unuse(inode))		if (!can_unuse(inode))
	continue;		continue;
	}		}
	hlist_del_init(&inode->i_hash);
	list_del_init(&inode->i_sb_list);
	list_move(&inode->i_list, &freeable);		list_move(&inode->i_list, &freeable);
	inode->i_state \|= I_FREEING;		inode->i_state \|= I_FREEING;
	nr_pruned++;		nr_pruned++;