Btrfs: fix potential race in extent buffer freeing
This sounds sort of impossible but it is the only thing I can think of and
at the very least it is theoretically possible so here it goes.
If we are in try_release_extent_buffer we will check that the ref count on
the extent buffer is 1 and not under IO, and then go down and clear the tree
ref. If between this check and clearing the tree ref somebody else comes in
and grabs a ref on the eb and the marks it dirty before
try_release_extent_buffer() does it's tree ref clear we can end up with a
dirty eb that will be freed while it is still dirty which will result in a
panic. Thanks,
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index e6243f7..e1939a6 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -4123,11 +4123,10 @@
* So bump the ref count first, then set the bit. If someone
* beat us to it, drop the ref we added.
*/
- if (!test_bit(EXTENT_BUFFER_TREE_REF, &eb->bflags)) {
+ spin_lock(&eb->refs_lock);
+ if (!test_and_set_bit(EXTENT_BUFFER_TREE_REF, &eb->bflags))
atomic_inc(&eb->refs);
- if (test_and_set_bit(EXTENT_BUFFER_TREE_REF, &eb->bflags))
- atomic_dec(&eb->refs);
- }
+ spin_unlock(&eb->refs_lock);
}
static void mark_extent_buffer_accessed(struct extent_buffer *eb)
@@ -4239,9 +4238,7 @@
goto free_eb;
}
/* add one reference for the tree */
- spin_lock(&eb->refs_lock);
check_buffer_tree_ref(eb);
- spin_unlock(&eb->refs_lock);
spin_unlock(&tree->buffer_lock);
radix_tree_preload_end();