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SubjectRe: [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@wunner.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

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