Re: [PATCH v2 1/1] lib/vsprintf: Fix %pfwf when current node refcount == 0

[Andy Shevchenko]

On Tue, Nov 14, 2023 at 03:35:58PM +0100, Herve Codina wrote:
> A refcount issue can appeared in __fwnode_link_del() due to the
> pr_debug() call:
>   WARNING: CPU: 0 PID: 901 at lib/refcount.c:25 refcount_warn_saturate+0xe5/0x110
>   Call Trace:
>   <TASK>
>   ...
>   of_node_get+0x1e/0x30
>   of_fwnode_get+0x28/0x40
>   fwnode_full_name_string+0x34/0x90
>   fwnode_string+0xdb/0x140
>   ...
>   vsnprintf+0x17b/0x630
>   ...
>   __fwnode_link_del+0x25/0xa0
>   fwnode_links_purge+0x39/0xb0
>   of_node_release+0xd9/0x180
>   ...
> 
> Indeed, an fwnode (of_node) is being destroyed and so, of_node_release()
> is called because the of_node refcount reached 0.
> From of_node_release() several function calls are done and lead to
> a pr_debug() calls with %pfwf to print the fwnode full name.
> The issue is not present if we change %pfwf to %pfwP.
> 
> To print the full name, %pfwf iterates over the current node and its
> parents and obtain/drop a reference to all nodes involved.
> 
> In order to allow to print the full name (%pfwf) of a node while it is
> being destroyed, do not obtain/drop a reference to this current node.

One nit-pick below, otherwise
Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>

...

>  	/* Loop starting from the root node to the current node. */
>  	for (depth = fwnode_count_parents(fwnode); depth >= 0; depth--) {
> -		struct fwnode_handle *__fwnode =
> -			fwnode_get_nth_parent(fwnode, depth);
> +		/*
> +		 * Only get a reference for other nodes (ie parents node).

"parent's node" (doesn't look right)? Or "parent nodes"?

> +		 * fwnode refcount may be 0 here.
> +		 */
> +		struct fwnode_handle *__fwnode = depth ?
> +			fwnode_get_nth_parent(fwnode, depth) : fwnode;
>  
>  		buf = string(buf, end, fwnode_get_name_prefix(__fwnode),
>  			     default_str_spec);
>  		buf = string(buf, end, fwnode_get_name(__fwnode),
>  			     default_str_spec);
>  
> -		fwnode_handle_put(__fwnode);
> +		if (depth)
> +			fwnode_handle_put(__fwnode);
>  	}

-- 
With Best Regards,
Andy Shevchenko