linux-kernel - Re: [PATCH 15/19] vfs: Add superblock notifications [ver #16]

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Message-ID: <1897788.1582295034@warthog.procyon.org.uk>
Date:   Fri, 21 Feb 2020 14:23:54 +0000
From:   David Howells <dhowells@...hat.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     dhowells@...hat.com, Al Viro <viro@...iv.linux.org.uk>,
        raven@...maw.net, Miklos Szeredi <mszeredi@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 15/19] vfs: Add superblock notifications [ver #16]

Jann Horn <jannh@...gle.com> wrote:

> > +               if (!s->s_watchers) {
> 
> READ_ONCE() ?

I'm not sure it matters.  It can only be set once, and the next time we read
it we're inside the lock.  And at this point, I don't actually dereference it,
and if it's non-NULL, it's not going to change.

> > +                       ret = add_watch_to_object(watch, s->s_watchers);
> > +                       if (ret == 0) {
> > +                               spin_lock(&sb_lock);
> > +                               s->s_count++;
> > +                               spin_unlock(&sb_lock);
> 
> Where is the corresponding decrement of s->s_count? I'm guessing that
> it should be in the ->release_watch() handler, except that there isn't
> one...

Um.  Good question.  I think this should do the job:

	static void sb_release_watch(struct watch *watch)
	{
		put_super(watch->private);
	}

And this then has to be set later:

	init_watch_list(wlist, sb_release_watch);

> > +       } else {
> > +               ret = -EBADSLT;
> > +               if (READ_ONCE(s->s_watchers)) {
> 
> (Nit: I don't get why you do a lockless check here before taking the
> lock - it'd be more straightforward to take the lock first, and it's
> not like you want to optimize for the case where someone calls
> sys_watch_sb() with invalid arguments...)

Fair enough.  I'll remove it.

> > +#ifdef CONFIG_SB_NOTIFICATIONS
> > +       if (unlikely(s->s_watchers)) {
> 
> READ_ONCE() ?

Shouldn't matter.  It's only read once and then a decision is made on it
immediately thereafter.  And if it's non-NULL, the value cannot change
thereafter.

David