linux-kernel - Re: [PATCH v2 2/2] cgroup, kernfs: Move cgroup to the RCU interface for name lookups

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Message-ID: <ZzTvsueQ_THOT6EW@slm.duckdns.org>
Date: Wed, 13 Nov 2024 08:28:02 -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 v2 2/2] cgroup, kernfs: Move cgroup to the RCU interface
 for name lookups

Hello,

On Wed, Nov 13, 2024 at 02:23:33PM +0100, Sebastian Andrzej Siewior wrote:
...
> > That is the easy part. kernfs_path_from_node() is different as it
> > requires the parent pointer. In order to distinguish the RCU from the
> > non-RCU version I need kernfs_root for the flag and depending on it, the
> > lock so the parent does not go away.
> > 
> > Would it work to add the pointer to kernfs_root into kernfs_node? This
> > would shrink kernfs_elem_dir by a pointer but the union would remain the
> > same size due to kernfs_elem_attr so the struct would grow.
> 
> The kernfs_node is released via RCU. That means if the RCU read section
> starts before kernfs_root() then we should always get a stable pointer,
> pointing to the same kernfs_root node since it is always the same one.
> Even if the `parent' pointer is replaced. Wouldn't we need __rcu
> annotation then for the `parent' pointer then?

Yeah, I think this is the better direction. Just make both the parent and
name RCU protected, drop the rename rwlock and use RCU deref for both
->parent and ->name so that the code path doesn't have to diverge.

Thanks.

-- 
tejun