Re: [PATCH] btrfs: fix refcount_t usage when deleting btrfs_delayed_nodes

[Liu Bo <bo.li.liu@oracle.com>]

On Fri, Dec 15, 2017 at 11:58:27AM -0800, Chris Mason wrote:
> refcounts have a generic implementation and an asm optimized one.  The
> generic version has extra debugging to make sure that once a refcount
> goes to zero, refcount_inc won't increase it.
> 
> The btrfs delayed inode code wasn't expecting this, and we're tripping
> over the warnings when the generic refcounts are used.  We ended up with
> this race:
> 
> Process A                                         Process B
>                                                   btrfs_get_delayed_node()
> 						  spin_lock(root->inode_lock)
> 						  radix_tree_lookup()
> __btrfs_release_delayed_node()
> refcount_dec_and_test(&delayed_node->refs)
> our refcount is now zero
> 						  refcount_add(2) <---
> 						  warning here, refcount
>                                                   unchanged
> 
> spin_lock(root->inode_lock)
> radix_tree_delete()
> 
> With the generic refcounts, we actually warn again when process B above
> tries to release his refcount because refcount_add() turned into a
> no-op.
> 
> We saw this in production on older kernels without the asm optimized
> refcounts.
> 
> The fix used here is to use refcount_inc_not_zero() to detect when the
> object is in the middle of being freed and return NULL.  This is almost
> always the right answer anyway, since we usually end up pitching the
> delayed_node if it didn't have fresh data in it.
> 
> This also changes __btrfs_release_delayed_node() to remove the extra
> check for zero refcounts before radix tree deletion.
> btrfs_get_delayed_node() was the only path that was allowing refcounts
> to go from zero to one.
> 

Reviewed-by: Liu Bo <bo.li.liu@oracle.com>

-liubo
> Signed-off-by: Chris Mason <clm@fb.com>
> Fixes: 6de5f18e7b0da
> cc: <stable@vger.kernel.org> #4.12+
> ---
>  fs/btrfs/delayed-inode.c | 45 ++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 34 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/btrfs/delayed-inode.c b/fs/btrfs/delayed-inode.c
> index 5d73f79..84c54af 100644
> --- a/fs/btrfs/delayed-inode.c
> +++ b/fs/btrfs/delayed-inode.c
> @@ -87,6 +87,7 @@ static struct btrfs_delayed_node *btrfs_get_delayed_node(
>  
>  	spin_lock(&root->inode_lock);
>  	node = radix_tree_lookup(&root->delayed_nodes_tree, ino);
> +
>  	if (node) {
>  		if (btrfs_inode->delayed_node) {
>  			refcount_inc(&node->refs);	/* can be accessed */
> @@ -94,9 +95,30 @@ static struct btrfs_delayed_node *btrfs_get_delayed_node(
>  			spin_unlock(&root->inode_lock);
>  			return node;
>  		}
> -		btrfs_inode->delayed_node = node;
> -		/* can be accessed and cached in the inode */
> -		refcount_add(2, &node->refs);
> +
> +		/* it's possible that we're racing into the middle of
> +		 * removing this node from the radix tree.  In this case,
> +		 * the refcount was zero and it should never go back
> +		 * to one.  Just return NULL like it was never in the radix
> +		 * at all; our release function is in the process of removing
> +		 * it.
> +		 *
> +		 * Some implementations of refcount_inc refuse to
> +		 * bump the refcount once it has hit zero.  If we don't do
> +		 * this dance here, refcount_inc() may decide to
> +		 * just WARN_ONCE() instead of actually bumping the refcount.
> +		 *
> +		 * If this node is properly in the radix, we want to
> +		 * bump the refcount twice, once for the inode
> +		 * and once for this get operation.
> +		 */
> +		if (refcount_inc_not_zero(&node->refs)) {
> +			refcount_inc(&node->refs);
> +			btrfs_inode->delayed_node = node;
> +		} else {
> +			node = NULL;
> +		}
> +
>  		spin_unlock(&root->inode_lock);
>  		return node;
>  	}
> @@ -254,17 +276,18 @@ static void __btrfs_release_delayed_node(
>  	mutex_unlock(&delayed_node->mutex);
>  
>  	if (refcount_dec_and_test(&delayed_node->refs)) {
> -		bool free = false;
>  		struct btrfs_root *root = delayed_node->root;
> +
>  		spin_lock(&root->inode_lock);
> -		if (refcount_read(&delayed_node->refs) == 0) {
> -			radix_tree_delete(&root->delayed_nodes_tree,
> -					  delayed_node->inode_id);
> -			free = true;
> -		}
> +		/*
> +		 * once our refcount goes to zero, nobody is allowed to
> +		 * bump it back up.  We can delete it now
> +		 */
> +		ASSERT(refcount_read(&delayed_node->refs) == 0);
> +		radix_tree_delete(&root->delayed_nodes_tree,
> +				  delayed_node->inode_id);
>  		spin_unlock(&root->inode_lock);
> -		if (free)
> -			kmem_cache_free(delayed_node_cache, delayed_node);
> +		kmem_cache_free(delayed_node_cache, delayed_node);
>  	}
>  }
>  
> -- 
> 2.9.5
> 
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