linux-kernel - Re: [PATCH v2 1/2] kernfs: Make it possible to use RCU for kernfs

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Message-ID: <ZzOj2z4g7nzWnCBb@slm.duckdns.org>
Date: Tue, 12 Nov 2024 08:52:11 -1000
From: Tejun Heo <tj@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	Michal Koutný <mkoutny@...e.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Hillf Danton <hdanton@...a.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Marco Elver <elver@...gle.com>, Zefan Li <lizefan.x@...edance.com>,
	tglx@...utronix.de
Subject: Re: [PATCH v2 1/2] kernfs: Make it possible to use RCU for
 kernfs_node::name lookup.

Hello,

On Tue, Nov 12, 2024 at 04:52:38PM +0100, Sebastian Andrzej Siewior wrote:
...
> KERNFS_ROOT_SAME_PARENT is added to signal that the parent never

Maybe KERNFS_ROOT_INVARIANT_PARENT captures it better?

...
> @@ -195,13 +191,47 @@ static int kernfs_path_from_node_locked(struct kernfs_node *kn_to,
>   */
>  int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen)
>  {
> +	struct kernfs_root *root;
>  
> +	guard(read_lock_irqsave)(&kernfs_rename_lock);
> +	if (kn) {
> +		root = kernfs_root(kn);
> +		if (WARN_ON_ONCE(root->flags & KERNFS_ROOT_SAME_PARENT))
> +			kn = NULL;

Hmm... does kn need to be set to NULL here?

> +	}
> +
> +	if (!kn)
> +		return strscpy(buf, "(null)", buflen);
> +
> +	return strscpy(buf, kn->parent ? kn->name : "/", buflen);
...
> +int kernfs_name_rcu(struct kernfs_node *kn, char *buf, size_t buflen)
> +{
> +	struct kernfs_root *root;
> +
> +	if (kn) {
> +		root = kernfs_root(kn);
> +		if (WARN_ON_ONCE(!(root->flags & KERNFS_ROOT_SAME_PARENT)))
> +			kn = NULL;

Ah, I suppose it's to keep things symmetric. That's fine.

> +	}
> +	if (!kn)
> +		return strscpy(buf, "(null)", buflen);
> +
> +	guard(rcu)();

Also, why are guards in different locations? Even when !SAME_PARENT, kn's
can't jump across roots, so guard there can also be in the same location as
this one?

...
> @@ -200,7 +205,10 @@ struct kernfs_node {
>  	 * parent directly.
>  	 */
>  	struct kernfs_node	*parent;
> -	const char		*name;
> +	union {
> +		const char		__rcu *name_rcu;
> +		const char		*name;
> +	};

Wouldn't it be simpler if ->name is always __rcu and !SAME_PARENT just
requires further protection on the read side?

Thanks.

-- 
tejun