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

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Date:   Wed, 28 Sep 2016 12:43:18 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
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 Tue, Sep 27, 2016 at 01:52:48PM +0200, Rafael J. Wysocki wrote:
> On Tue, Sep 27, 2016 at 10:54 AM, Lukas Wunner <lukas@...ner.de> wrote:
> > 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;
> >> +             }
> >> +             link->status = DEVICE_LINK_SUPPLIER_UNBIND;
> >
> > 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.
> 
> But the consumer is unbound only after setting the link status to
> DEVICE_LINK_SUPPLIER_UNBIND and then it won't probe again.

Sorry, looking at the code with a fresh pair of eyeballs I realize the
scenario for the infinite loop is different from what I've written above:
The infinite loop can occur if the consumer probes continuously but never
succeeds, e.g. due to some unfulfilled condition in its ->probe hook.

That could be fixed by moving the assignment

		link->status = DEVICE_LINK_SUPPLIER_UNBIND;

above the preceding if-block (but below "status = link->status;").

The next time the consumer probes, it will return with -EPROBE_DEFER
(return value of device_links_check_suppliers()).

However the semantics of DEVICE_LINK_SUPPLIER_UNBIND are "consumer not
bound and blocked from probing", with the above change it would become
"consumer may or may not be bound and blocked from probing".

Thus it would also be necessary to change device_links_driver_bound()
so that it doesn't update the status to DEVICE_LINK_ACTIVE.  Also,
device_links_busy() and device_links_unbind_consumers() would have
to check boundness with device_is_bound() if the status is
DEVICE_LINK_SUPPLIER_UNBIND.  Perhaps it would be easier to add
separate link states for this, or perhaps this problem is too
theoretical to bother dealing with it.

Thanks,

Lukas