Btrfs: only use the existing eb if it's count isn't 0 (115391d2) · Commits · 戴 / test

We can run into a problem where we find an eb for our existing page already on the radix tree but it has a ref count of 0. It hasn't yet been removed by RCU yet so this can cause issues where we will use the EB after free. So do atomic_inc_not_zero on the exists->refs and if it is zero just do synchronize_rcu() and try again. We won't have to worry about new allocators coming in since they will block on the page lock at this point. Thanks, Signed-off-by:

Josef Bacik <josef@redhat.com>

fs/btrfs/extent_io.c

+8 −2

Original line number	Diff line number	Diff line
		@@ -3750,7 +3750,7 @@ struct extent_buffer alloc_extent_buffer(struct extent_io_tree tree,
		}
		if (uptodate)
		set_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags);

		again:
		ret = radix_tree_preload(GFP_NOFS & ~__GFP_HIGHMEM);
		if (ret)
		goto free_eb;
		@@ -3760,7 +3760,13 @@ struct extent_buffer alloc_extent_buffer(struct extent_io_tree tree,
		if (ret == -EEXIST) {
		exists = radix_tree_lookup(&tree->buffer,
		start >> PAGE_CACHE_SHIFT);
		atomic_inc(&exists->refs);
		if (!atomic_inc_not_zero(&exists->refs)) {
		spin_unlock(&tree->buffer_lock);
		radix_tree_preload_end();
		synchronize_rcu();
		exists = NULL;
		goto again;
		}
		spin_unlock(&tree->buffer_lock);
		radix_tree_preload_end();
		goto free_eb;

Admin message