linux-kernel - Re: [PATCH 2/2] drivers: core: Remove glue dirs from sysfs earlier

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Date:   Wed, 04 Jul 2018 11:10:23 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Joel Stanley <joel@....id.au>
Subject: Re: [PATCH 2/2] drivers: core: Remove glue dirs from sysfs earlier

On Tue, 2018-07-03 at 08:46 -0700, Tejun Heo wrote:
> Hello,
> 
> On Tue, Jul 03, 2018 at 03:22:47PM +1000, Benjamin Herrenschmidt wrote:
> > +bool kernfs_has_children(struct kernfs_node *kn)
> > +{
> > +	bool has_children = false;
> > +	struct kernfs_node *pos;
> > +
> > +	/* Lockless shortcut */
> > +	if (RB_EMPTY_NODE(&kn->rb))
> > +		return false;
> 
> Hmm... shouldn't it be testing !rb_first(kn->dir.children)?  The above
> would test whether @kn itself is unlinked.

Ah possibly, the rb tree usage got me confused a few times in there.

> > +
> > +	/* Now check for active children */
> > +	mutex_lock(&kernfs_mutex);
> > +	pos = NULL;
> > +	while ((pos = kernfs_next_descendant_post(pos, kn)) && pos != kn) {
> > +		if (kernfs_active(pos)) {
> > +			has_children = true;
> > +			break;
> > +		}
> > +	}
> > +	mutex_unlock(&kernfs_mutex);
> > +
> > +	return has_children;
> 
> The active ref is there to synchronize removal against in-flight reads
> so that kernfs_remove() can wait for them to drain.  On return from
> kernfs_remove(), the node is guaranteed to be off the rbtree and the
> above test isn't necessary.  !rb_first() test should be enough
> (provided that there's external synchronization, of course).

Ok. Yes there's external synchronization. So a simple !rb_first test
doesn't need the mutex I suppose ?

Cheers,
Ben.

> 
> Thanks.
>