linux-kernel - Re: [PATCH v6 4/5] PM / runtime: Use device links

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Date:   Sun, 18 Dec 2016 17:37:25 +0100
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: [PATCH v6 4/5] PM / runtime: Use device links

On Sun, Dec 18, 2016 at 04:53:26PM +0100, Rafael J. Wysocki wrote:
> On Sun, Dec 18, 2016 at 3:01 PM, Lukas Wunner <lukas@...ner.de> wrote:
> > Hi Rafael,
> >
> > spotted what looks like a bug in the device links runtime PM code:
> >
> > When resuming a device, __rpm_callback() calls rpm_get_suppliers(dev):
> >
> >> +                     retval = rpm_get_suppliers(dev);
> >> +                     if (retval)
> >> +                             goto fail;
> >
> >
> > This will walk the list of suppliers and call pm_runtime_get_sync()
> > for each of them:
> >
> >> +static int rpm_get_suppliers(struct device *dev)
> >> +{
> >> +     struct device_link *link;
> >> +
> >> +     list_for_each_entry_rcu(link, &dev->links.suppliers, c_node) {
> >> +             int retval;
> > [...]
> >> +             retval = pm_runtime_get_sync(link->supplier);
> >> +             if (retval < 0) {
> >> +                     pm_runtime_put_noidle(link->supplier);
> >> +                     return retval;
> >
> >
> > If pm_runtime_get_sync() failed, e.g. because runtime PM is disabled
> > on a supplier, the function will put the reference of the failed
> > supplier and return.
> >
> > Back in __rpm_callback() we jump to the fail mark, where we call
> > rpm_put_suppliers().
> >
> >> + fail:
> >> +                     rpm_put_suppliers(dev);
> >> +
> >> +                     device_links_read_unlock(idx);
> >
> >
> > This walks the list of suppliers and releases a ref for each of them:
> >
> >> +static void rpm_put_suppliers(struct device *dev)
> >> +{
> >> +     struct device_link *link;
> >> +
> >> +     list_for_each_entry_rcu(link, &dev->links.suppliers, c_node)
> >> +             if (link->rpm_active &&
> >> +                 READ_ONCE(link->status) != DL_STATE_SUPPLIER_UNBIND) {
> >> +                     pm_runtime_put(link->supplier);
> >> +                     link->rpm_active = false;
> >> +             }
> >> +}
> >
> >
> > This looks wrong:  We've already put a ref on the failed supplier, so here
> > we're putting another one.
> 
> Are we?  I would think link->rpm_active would be false for the failed
> one, wouldn't it?

Ah, so link->rpm_active means the consumer is holding a ref on the supplier.
Missed that, sorry for the false alarm and thanks for the clarification.

Lukas