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

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Message-ID: <YoKna/Ty+k9O+pjt@slm.duckdns.org>
Date:   Mon, 16 May 2022 09:35:07 -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 Mon, May 16, 2022 at 02:00:50AM +1000, Imran Khan wrote:
> +/*
> + * Deref RCU protected kn->attr.open.
> + * If both @of->list and @kn->attr.open->files are non empty, we can safely
> + * assume that @of is on @kn->attr.open and hence @kn->attr.open will not
> + * vanish and derefeencing is safe here.
> + */

Maybe use proper function comment starting with /**?

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

Why does it need to return NULL on empty on->files? We wanna trigger lockdep
warning cuz that's a bug but it's not like the caller can recover from it
(short of causing unexpected user visible error), so I don't see what the
point is.

> If this looks okay then I can replace usage of kernfs_deref_on_raw with
> kernfs_deref_on_check.

So, this is the main deref function without holding the mutex, right? Then
name it kernfs_deref_open_node() (or on but it seem a bit confusing to me).

> > 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.
> > 
> 
> Since ->attr.open is always accessed/modified under kernfs_open_file_mutex, I
> have included this check in helper which should be used only while holding this
> mutex. Do you mean that I should include some additional checks as well in the
> below helper:

Yeah, you're right. _protected makes sense for this one.

Thanks.

-- 
tejun