linux-kernel - Re: [PATCH] kernfs: Use RCU for kernfs

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Message-ID: <ZzOmXbpO3apRH0Cz@slm.duckdns.org>
Date: Tue, 12 Nov 2024 09:02:53 -1000
From: Tejun Heo <tj@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Boqun Feng <boqun.feng@...il.com>, Hillf Danton <hdanton@...a.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Marco Elver <elver@...gle.com>, linux-kernel@...r.kernel.org,
	gregkh@...uxfoundation.org, tglx@...utronix.de,
	syzbot <syzbot+6ea37e2e6ffccf41a7e6@...kaller.appspotmail.com>
Subject: Re: [PATCH] kernfs: Use RCU for kernfs_node::name lookup.

Hello, Sebastian.

Sorry about late reply. Have been sick for a bit.

On Mon, Nov 11, 2024 at 06:04:50PM +0100, Sebastian Andrzej Siewior wrote:
...
> Let me check if I understood. We have three users of kernfs:
> 
> - cgroup
>   cgroup1_rename(): parent check (would get the new KERNFS_ROOT flag)
> 
> - resctrl
>   rdtgroup_rename(): seems to allow any "mon group" directory
>   different parent possible.
> 
> - sysfs
>   sysfs_move_dir_ns(): reame to a different parent

I'm not sure about resctrl but here we just need to add that flag to cgroup,
right?

> That new RCU interface would be used by cgroup only and sysfs/ resctrl
> would remain using the "old" code?
> If so, would you  prefer 
> |struct kernfs_node {
> |	…
> |	union {
> |		const char              name;
> |		const char              __rcu *name_rcu;
> |	}
> 
> to avoid patching resctrl + sysfs for for the rcu_derference name
> lookup?

As replied on the patches, it probably is cleaner to always use __rcu and
apply the additional locking on the reader side if renaming across different
parents is allowed.

Thanks.

-- 
tejun