linux-kernel - Re: [PATCH v5 6/6] kernfs: Use RCU to access kernfs

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Message-ID: <Z5lAqVmFkMoFACae@slm.duckdns.org>
Date: Tue, 28 Jan 2025 10:40:09 -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>, tglx@...utronix.de,
	syzbot+6ea37e2e6ffccf41a7e6@...kaller.appspotmail.com
Subject: Re: [PATCH v5 6/6] kernfs: Use RCU to access kernfs_node::name.

On Tue, Jan 28, 2025 at 09:42:26AM +0100, Sebastian Andrzej Siewior wrote:
> Using RCU lifetime rules to access kernfs_node::name can avoid the
> trouble kernfs_rename_lock in kernfs_name() and kernfs_path_from_node()
> if the fs was created with KERNFS_ROOT_INVARIANT_PARENT. This is useful
> as it allows to implement kernfs_path_from_node() only with RCU
> protection and avoiding kernfs_rename_lock. The lock is only required if
> the __parent node can be changed and the function requires an unchanged
> hierarchy while it iterates from the node to its parent.

A short mention of how avoiding kernfs_rename_lock matters would be great -
ie. where did this show up?

> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index 955999aecfca9..cc83dbc70a8ef 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -505,6 +505,11 @@ int parse_bw(struct rdt_parse_data *data, struct resctrl_schema *s,
>  
>  extern struct mutex rdtgroup_mutex;
>  
> +static inline const char *rdt_kn_get_name(const struct kernfs_node *kn)
> +{
> +	return rcu_dereference_check(kn->name, lockdep_is_held(&rdtgroup_mutex));
> +}

Maybe rdt_kn_name()?

Other than those,

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun