linux-kernel - Re: [PATCH RESEND v4 1/4] kernfs: make ->attr.open RCU protected.

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Message-ID: <YqalHuEB6OjjUosV@slm.duckdns.org>
Date:   Sun, 12 Jun 2022 16:46:54 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Imran Khan <imran.f.khan@...cle.com>
Cc:     viro@...iv.linux.org.uk, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v4 1/4] kernfs: make ->attr.open RCU protected.

Hello,

On Mon, Jun 13, 2022 at 12:36:12PM +1000, Imran Khan wrote:
> >> +static struct kernfs_open_node *
> >> +kernfs_check_open_node_protected(struct kernfs_node *kn)
> >> +{
> >> +	return rcu_dereference_check(kn->attr.open,
> >> +				      lockdep_is_held(&kernfs_open_file_mutex));
> >> +}
> > 
> > I don't understand why the above is necessary. Whether you're a reader or
> > writer, you can deref the pointer w/ _protected as long as you're holding
> > the lock, right?
> 
> As per [1], we should use rcu_dereference_check() for the reader side when we
> are holding the lock.

Hmm.... can you quote the exact phrase that you took the above from?

Thanks.

-- 
tejun