linux-kernel - Re: [PATCH 2/5] kernfs: make ->attr.open RCU protected.

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Message-ID: <Yn0/RrgMRie2YPEb@slm.duckdns.org>
Date:   Thu, 12 May 2022 07:09:26 -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 2/5] kernfs: make ->attr.open RCU protected.

Hello,

On Fri, May 06, 2022 at 09:12:23AM +1000, Imran Khan wrote:
> On 6/5/22 6:01 am, Tejun Heo wrote:
> > On Thu, Apr 28, 2022 at 03:54:28PM +1000, Imran Khan wrote:
> >> +static struct kernfs_open_node *kernfs_deref_on_raw(struct kernfs_node *kn)
> >> +{
> >> +	return rcu_dereference_raw(kn->attr.open);
> >> +}
> > 
> > Wrapping the above probably isn't helping anything.
> > 
> 
> This change is using raw deref at a few places, so I thought of putting
> raw deref under a wrapper and explain in the wrapper function comment
> under what conditions raw dereferencing was safe.

I don't think they need raw deref in the first place and if you *really*
need raw deref, the wrapper explaining why they're needed and how they're
safe is the whole point of it and I don't think the wrapper achieves that.

> >> +/*
> >> + * Check ->attr.open corresponding to @kn while holding kernfs_open_file_mutex.
> >> + * ->attr.open is modified under kernfs_open_file_mutex. So it can be safely
> >> + * accessed outside RCU read-side critical section, while holding the mutex.
> >> + */
> >> +static struct kernfs_open_node *kernfs_check_on_protected(struct kernfs_node *kn)
> >> +{
> >> +	return rcu_dereference_check(kn->attr.open,
> >> +				      lockdep_is_held(&kernfs_open_file_mutex));
> >> +}
> > 
> > Maybe name this just kernfs_deref_on()?
> > 
> 
> Sure. I can mention in the function comment that this should be used
> only under kernfs_open_file_mutex.

and in the check condition, add the conditions that you need to make this
not trigger warning when used in all the places that you wanna use it.

> +static struct kernfs_open_node *kernfs_deref_on_raw(struct
> kernfs_open_file *of, struct kernfs_node *kn)
>  {
> -       return rcu_dereference_raw(kn->attr.open);
> +       struct kernfs_open_node *on;
> +
> +       if (list_empty(&of->list))
> +               return NULL;
> +
> +       on = rcu_dereference_raw(kn->attr.open);
> +
> +       if (list_empty(&on->files))
> +               return NULL;
> +       else
> +               return on;

Put the above conditions in the rcu_dereference_check(). That's what it is
for - describing the additional conditions that make the dereference safe.

Thanks.

-- 
tejun