Commit 4120db47 authored by Artem B. Bityuckiy's avatar Artem B. Bityuckiy Committed by Linus Torvalds
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[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: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 168a9fd6
......@@ -282,6 +282,13 @@ static void dispose_list(struct list_head *head)
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++;
}
......@@ -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);
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++;
......@@ -439,8 +444,6 @@ 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++;
......
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