linux-kernel - Re: [PATCH v6 0/6] kernfs: Use RCU to access kernfs

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Message-ID: <Z5vGqAxiaL_weoWU@slm.duckdns.org>
Date: Thu, 30 Jan 2025 08:36:24 -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
Subject: Re: [PATCH v6 0/6] kernfs: Use RCU to access
 kernfs_node::{parent|name}.

On Thu, Jan 30, 2025 at 03:02:01PM +0100, Sebastian Andrzej Siewior wrote:
> Hi,
> 
> This started as a bug report by Hillf Danton and aims to access
> kernfs_node::{name|parent} with RCU to avoid the lock during
> kernfs_path_from_node().
> 
> I've split the individual fixes in separate patches (#1 to #4). I've
> also split the ::parent and ::name RCU conversation into a single patch
> (#5 and #6).
> 
> v5…v6 https://lore.kernel.org/all/20250128084226.1499291-1-bigeasy@linutronix.de/
>   - s/rdt_kn_get_name/rdt_kn_name/
>   - s/rdt_get_kn_parent_priv/rdt_kn_parent_priv/
>   - s/kn_get_priv/kn_priv/
>   - The comment, that has been removed in kernfs_put(), is back.
>   - Using rcu_access_pointer() in kernfs_activate_one() and kernfs_dir_pos()
>     instead of kernfs_parent() where the pointer is not dereferenced but
>     just compared.
> 
> v4…v5 https://lore.kernel.org/all/20250124174614.866884-1-bigeasy@linutronix.de/
>   - rdtgroup:
>     - Add a comment to rdt_get_kn_parent_priv() regarding lifetime of
>       parent.
>     - Move individual rcu_dereference_check() invocations into
>       rdt_kn_parent() with a comment on lifetime.
>     - Use rcu_access_pointer() in kernfs_to_rdtgroup() instead
>       rcu_dereference_check(, true)
>   - s/kernfs_rcu_get_name/kernfs_rcu_name/
>   - Move all rcu_dereference_check() within kernfs into kernfs_parent()
>     and extend its checks to have all cases in one spot. Document why
>     each case makes sense.
>   - kernfs_notify_workfn(): Do unlocks in the reverse order of locks.
>   - Add kernfs_root_flags() and use it in cgroup's kn_get_priv() to
>     check the right KERNFS_ROOT_INVARIANT_PARENT flag.
> 
> v3: https://lore.kernel.org/all/20241121175250.EJbI7VMb@linutronix.de/
> v2: https://lore.kernel.org/all/20241112155713.269214-1-bigeasy@linutronix.de/
> v1: https://lore.kernel.org/all/20241108222406.n5azgO98@linutronix.de/

Whole series looks good to me.

Thanks.

-- 
tejun