netdev - Re: [patch net-next] devlink: don't take instance lock for nested handle put

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Date: Thu, 5 Oct 2023 18:30:29 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Jiri Pirko <jiri@...nulli.us>
Cc: netdev@...r.kernel.org, pabeni@...hat.com, davem@...emloft.net,
 edumazet@...gle.com, gal@...dia.com
Subject: Re: [patch net-next] devlink: don't take instance lock for nested
 handle put

On Tue,  3 Oct 2023 09:43:49 +0200 Jiri Pirko wrote:
> To fix this, don't take the devlink instance lock when putting nested
> handle. Instead, rely on devlink reference to access relevant pointers
> within devlink structure. Also, make sure that the device does

struct device ?

> not disappear by taking a reference in devlink_alloc_ns().

> @@ -310,6 +299,7 @@ static void devlink_release(struct work_struct *work)
>  
>  	mutex_destroy(&devlink->lock);
>  	lockdep_unregister_key(&devlink->lock_key);
> +	put_device(devlink->dev);

IDK.. holding references until all references are gone may lead 
to reference cycles :(

>  	kfree(devlink);
>  }

> @@ -92,9 +93,8 @@ int devlink_nl_put_nested_handle(struct sk_buff *msg, struct net *net,
>  		return -EMSGSIZE;
>  	if (devlink_nl_put_handle(msg, devlink))
>  		goto nla_put_failure;
> -	if (!net_eq(net, devlink_net(devlink))) {
> -		int id = peernet2id_alloc(net, devlink_net(devlink),
> -					  GFP_KERNEL);
> +	if (!net_eq(net, devl_net)) {
> +		int id = peernet2id_alloc(net, devl_net, GFP_KERNEL);
>  
>  		if (nla_put_s32(msg, DEVLINK_ATTR_NETNS_ID, id))
>  			return -EMSGSIZE;

Looks like pure refactoring. But are you sure that the netns can't
disappear? We're not holding the lock, the instance may get moved.

Overall I feel like recording the references on the objects will be
an endless source of locking pain. Would it be insane if we held 
the relationships as independent objects? Not as attributes of either
side?