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

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Date:   Sun, 12 Jun 2022 07:58:37 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Imran Khan <imran.f.khan@...cle.com>
Cc:     viro@...iv.linux.org.uk, gregkh@...uxfoundation.org,
        matthew.wilcox@...cle.com, konrad.wilk@...cle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v4 1/4] kernfs: make ->attr.open RCU protected.

Hello,

Sorry about the long delay.

On Thu, Jun 02, 2022 at 04:39:04PM +1000, Imran Khan wrote:
> +/**
> + * kernfs_deref_open_node_protected - Get kernfs_open_node corresponding to @kn
> + *
> + * @kn: target kernfs_node.
> + *
> + * Fetch and return ->attr.open of @kn when caller(writer) holds
> + * kernfs_open_file_mutex.
> + *
> + * Update of ->attr.open happens under kernfs_open_file_mutex. So as long as
> + * the current updater (caller) is holding this mutex, other updaters will not
> + * be able to change ->attr.open and this means that we can safely deref
> + * ->attr.open outside RCU read-side critical section.
> + *
> + * This should ONLY be used by updaters of ->attr.open and caller needs to make
> + * sure that kernfs_open_file_mutex is held.
> + */
> +static struct kernfs_open_node *
> +kernfs_deref_open_node_protected(struct kernfs_node *kn)
> +{
> +	return rcu_dereference_protected(kn->attr.open,
> +				lockdep_is_held(&kernfs_open_file_mutex));
> +}
> +
> +/**
> + * kernfs_check_open_node_protected - Get kernfs_open_node corresponding to @kn
> + *
> + * @kn: target kernfs_node.
> + *
> + * Fetch and return ->attr.open of @kn when caller(reader) holds
> + * kernfs_open_file_mutex.
> + *
> + * Update of ->attr.open happens under kernfs_open_file_mutex. So as long as
> + * the current reader (caller) is holding this mutex, updaters will not be
> + * able to change ->attr.open and this means that we can safely deref
> + * ->attr.open outside RCU read-side critical section.
> + *
> + * This should ONLY be used by readers of ->attr.open and caller needs to make
> + * sure that kernfs_open_file_mutex is held.
> + */
> +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? If I'm mistaken and somehow a reader needs to use a
deref_check, I don't see a reason for this to be a separate accessor. Why
not just merge the condition into the kernfs_deref_open_node()?

Thanks.

-- 
tejun