Re: [PATCH][v2] btrfs: use refcount_inc_not_zero in kill_all_nodes

[Nikolay Borisov <nborisov@suse.com>]

On 26.09.19 г. 15:08 ч., Josef Bacik wrote:
> We hit the following warning while running down a different problem
> 
> [ 6197.175850] ------------[ cut here ]------------
> [ 6197.185082] refcount_t: underflow; use-after-free.
> [ 6197.194704] WARNING: CPU: 47 PID: 966 at lib/refcount.c:190 refcount_sub_and_test_checked+0x53/0x60
> [ 6197.521792] Call Trace:
> [ 6197.526687]  __btrfs_release_delayed_node+0x76/0x1c0
> [ 6197.536615]  btrfs_kill_all_delayed_nodes+0xec/0x130
> [ 6197.546532]  ? __btrfs_btree_balance_dirty+0x60/0x60
> [ 6197.556482]  btrfs_clean_one_deleted_snapshot+0x71/0xd0
> [ 6197.566910]  cleaner_kthread+0xfa/0x120
> [ 6197.574573]  kthread+0x111/0x130
> [ 6197.581022]  ? kthread_create_on_node+0x60/0x60
> [ 6197.590086]  ret_from_fork+0x1f/0x30
> [ 6197.597228] ---[ end trace 424bb7ae00509f56 ]---
> 
> This is because the free side drops the ref without the lock, and then
> takes the lock if our refcount is 0.  So you can have nodes on the tree
> that have a refcount of 0.  Fix this by zero'ing out that element in our
> temporary array so we don't try to kill it again.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Looks good, one minor nit below though and you can add:

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

> ---
> v1->v2:
> - I'm an idiot.
> 
>  fs/btrfs/delayed-inode.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/btrfs/delayed-inode.c b/fs/btrfs/delayed-inode.c
> index 1f7f39b10bd0..81b2fd46886f 100644
> --- a/fs/btrfs/delayed-inode.c
> +++ b/fs/btrfs/delayed-inode.c
> @@ -1948,13 +1948,16 @@ void btrfs_kill_all_delayed_nodes(struct btrfs_root *root)
>  			break;
>  		}
>  
> -		inode_id = delayed_nodes[n - 1]->inode_id + 1;
> -
> -		for (i = 0; i < n; i++)
> -			refcount_inc(&delayed_nodes[i]->refs);
> +		for (i = 0; i < n; i++) {
> +			inode_id = delayed_nodes[i]->inode_id + 1;

Since you no longer are doing a break in the loop there is no point in
assigning inode_id  in the loop. You can  retain the old code that does:
inode_id = delayed_nodes[n - 1]->inode_id + 1;

Since with the current structure it's always guaranteed that inode_id
will be 1 higher than the inode_Id of the last inode in delayed_nodes .

> +			if (!refcount_inc_not_zero(&delayed_nodes[i]->refs))
> +				delayed_nodes[i] = NULL;
> +		}
>  		spin_unlock(&root->inode_lock);
>  
>  		for (i = 0; i < n; i++) {
> +			if (!delayed_nodes[i])
> +				continue;
>  			__btrfs_kill_delayed_node(delayed_nodes[i]);
>  			btrfs_release_delayed_node(delayed_nodes[i]);
>  		}
>