Re: [PATCH V2] bcache: fix potential deadlock problem in btree_gc_coalesce

[Coly Li <colyli@suse.de>]

On 2020/4/26 16:06, Zhiqiang Liu wrote:
> From: Zhiqiang Liu <liuzhiqiang26@huawei.com>
> 
> coccicheck reports:
>   drivers/md//bcache/btree.c:1538:1-7: preceding lock on line 1417
> 
> btree_gc_coalesce func is designed to coalesce two adjacent nodes in
> new_nodes[GC_MERGE_NODES] and finally release one node. All nodes`write_lock,
> new_nodes[i]->write_lock, are holded before coalescing adjacent nodes,
> and them will be released after coalescing successfully.
> 
> However, if the coalescing process fails, such as no enough space of new_nodes[1]
> to fit all of the remaining keys in new_nodes[0] and realloc keylist failed, we
> will goto to out_nocoalesce tag directly without releasing new_nodes[i]->write_lock.
> Then, a deadlock will occur after calling btree_node_free to free new_nodes[i],
> which also try to acquire new_nodes[i]->write_lock.
> 
> Here, we add a new tag 'out_unlock_nocoalesce' before out_nocoalesce tag to release
> new_nodes[i]->write_lock when coalescing process fails.
> 
> --
> V1->V2: rewrite commit log (suggested by Coly Li) and rename the patch
> 
> Fixes: 2a285686c1 ("bcache: btree locking rework")
> Signed-off-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>

OK, I will add it to my for-test queue.

Thanks.

Coly Li


> ---
>  drivers/md/bcache/btree.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
> index fa872df4e770..cad8b0b97e33 100644
> --- a/drivers/md/bcache/btree.c
> +++ b/drivers/md/bcache/btree.c
> @@ -1447,7 +1447,7 @@ static int btree_gc_coalesce(struct btree *b, struct btree_op *op,
>  			if (__set_blocks(n1, n1->keys + n2->keys,
>  					 block_bytes(b->c)) >
>  			    btree_blocks(new_nodes[i]))
> -				goto out_nocoalesce;
> +				goto out_unlock_nocoalesce;
> 
>  			keys = n2->keys;
>  			/* Take the key of the node we're getting rid of */
> @@ -1476,7 +1476,7 @@ static int btree_gc_coalesce(struct btree *b, struct btree_op *op,
> 
>  		if (__bch_keylist_realloc(&keylist,
>  					  bkey_u64s(&new_nodes[i]->key)))
> -			goto out_nocoalesce;
> +			goto out_unlock_nocoalesce;
> 
>  		bch_btree_node_write(new_nodes[i], &cl);
>  		bch_keylist_add(&keylist, &new_nodes[i]->key);
> @@ -1522,6 +1522,10 @@ static int btree_gc_coalesce(struct btree *b, struct btree_op *op,
>  	/* Invalidated our iterator */
>  	return -EINTR;
> 
> +out_unlock_nocoalesce:
> +	for (i = 0; i < nodes; i++)
> +		mutex_unlock(&new_nodes[i]->write_lock);
> +
>  out_nocoalesce:
>  	closure_sync(&cl);
>