linux-kernel - Re: [Update][RFC/RFT][PATCH v3 2/5] driver core: Functional dependencies tracking support

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Message-ID: <20160927085413.GA5744@wunner.de>
Date:   Tue, 27 Sep 2016 10:54:13 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM list <linux-pm@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Mark Brown <broonie@...nel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kevin Hilman <khilman@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        "Luis R. Rodriguez" <mcgrof@...e.com>
Subject: Re: [Update][RFC/RFT][PATCH v3 2/5] driver core: Functional
 dependencies tracking support

On Fri, Sep 16, 2016 at 02:33:55PM +0200, Rafael J. Wysocki wrote:
> +void device_links_unbind_consumers(struct device *dev)
> +{
> +	struct device_link *link;
> +	int idx;
> +
> + start:
> +	idx = device_links_read_lock();
> +
> +	list_for_each_entry_rcu(link, &dev->links_to_consumers, s_node) {
> +		enum device_link_status status;
> +
> +		if (link->flags & DEVICE_LINK_STATELESS)
> +			continue;
> +
> +		spin_lock(&link->lock);
> +		status = link->status;
> +		if (status == DEVICE_LINK_CONSUMER_PROBE) {
> +			spin_unlock(&link->lock);
> +
> +			device_links_read_unlock(idx);
> +
> +			wait_for_device_probe();
> +			goto start;
> +		}

While revisiting this function it just occurred to me that there's
a theoretical infinite loop here if the consumer probes, is unbound
by the supplier, then reprobes again before the supplier had a chance
to update the link to DEVICE_LINK_SUPPLIER_UNBIND.  Perhaps this isn't
a problem in practice, but noting anyway.

The problem is that the link state is written to both by the supplier
and consumer.  If there was a separate bit in struct device_link to
indicate the supplier's desire to unbind, the problem would go away.
However a mix of such a bit plus the state machine would become
somewhat confusing...

Best regards,

Lukas