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Commit 594831c4 authored by Josef Bacik's avatar Josef Bacik Committed by Chris Mason
Browse files

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: default avatarJosef Bacik <jbacik@fusionio.com>
parent e64860aa

fs/btrfs/extent_io.c

+3 −6
Original line number Diff line number Diff line
@@ -4123,11 +4123,10 @@ static void check_buffer_tree_ref(struct extent_buffer *eb) 
	 * 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 @@ again: 
		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();