linux-kernel - Re: [PATCH v3] kernfs: Use RCU for kernfs

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Message-ID: <q77njpa2bvo52lvlu47fa7tlkqivqwf2mwudxycsxqhu2mf35s@ye4i3gsy4bl7>
Date: Mon, 25 Nov 2024 15:49:34 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org, 
	"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>, Tejun Heo <tj@...nel.org>, 
	Zefan Li <lizefan.x@...edance.com>, tglx@...utronix.de, 
	syzbot+6ea37e2e6ffccf41a7e6@...kaller.appspotmail.com
Subject: Re: [PATCH v3] kernfs: Use RCU for kernfs_node::name and ::parent
 lookup.

Hello.

On Thu, Nov 21, 2024 at 06:52:50PM GMT, Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:

> - kernfs_rename_ns() is only using kernfs_rename_lock if the parents are
>   different. All users users use either RCU or kernfs_rwsem.
> - kernfs_fop_readdir() drops kernfs_root::kernfs_rwsem while holding a
>   reference to name and invoking dir_emit(). This has been changed and
>   lock is held.
> - kernfs_notify_workfn() access kernfs_node::name without any
>   protection. Added kernfs_root::kernfs_rwsem for the iteration.
> - kernfs_get_parent_dentry() acquires now kernfs_root::kernfs_rwsem
>   while accessing the parent node.
> - kernfs_node_dentry() acquires now kernfs_root::kernfs_rwsem while
>   parent is accessed and the name looked up.

Why is the kernfs_root::kernfs_rwsem newly R-taken? Shouldn't be RCU
read section sufficient for those users?

(Perhaps it's related to second observation I have -- why there is
sometimes kernfs_rcu_get_parent() whereas there are other call sites
with mere rcu_dereference(kn->parent)?)



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