Re: [PATCH v1] PM-runtime: Check supplier_preactivated before release supplier

[Peter Wang]

On 6/30/22 12:01 AM, Rafael J. Wysocki wrote:
> [Add CCs to linix-pm, LKML and Greg]
>
> On Wednesday, June 29, 2022 5:32:00 PM CEST Rafael J. Wysocki wrote:
> > On Wed, Jun 29, 2022 at 4:47 PM Peter Wang <peter.wang@xxxxxxxxxxxx> wrote:
> > > On 6/29/22 9:22 PM, Rafael J. Wysocki wrote:
> > > > On Wed, Jun 29, 2022 at 5:02 AM Peter Wang <peter.wang@xxxxxxxxxxxx> wrote:
> > > > > On 6/28/22 11:54 PM, Rafael J. Wysocki wrote:
> > > > > > On Tue, Jun 28, 2022 at 3:53 AM Peter Wang <peter.wang@xxxxxxxxxxxx> wrote:
> > > > > > > On 6/28/22 3:00 AM, Rafael J. Wysocki wrote:
> > > > > > > > On Mon, Jun 13, 2022 at 2:08 PM <peter.wang@xxxxxxxxxxxx> wrote:
> > > > > > > > > From: Peter Wang <peter.wang@xxxxxxxxxxxx>
> > > > > > > > >
> > > > > > > > > With divice link of DL_FLAG_PM_RUNTIME, if consumer call pm_runtime_get_suppliers
> > > > > > > > > to prevent supplier enter suspend, pm_runtime_release_supplier should
> > > > > > > > > check supplier_preactivated before let supplier enter suspend.
> > > > > > > > Why?
> > > > > > > because supplier_preactivated is true means supplier cannot enter
> > > > > > > suspend, right?
> > > > > > No, it doesn't mean that.
> > > > > Hi Rafael,
> > > > >
> > > > > if supplier_preactivated is true, means someone call
> > > > > pm_runtime_get_suppliers and
> > > > > before pm_runtime_put_suppliers right? This section suppliers should not
> > > > > enter suspend.
> > > > No, this is not how this is expected to work.
> > > >
> > > > First off, the only caller of pm_runtime_get_suppliers() and
> > > > pm_runtime_put_suppliers() is __driver_probe_device().  Really nobody
> > > > else has any business that would require calling them.
> > > Hi Rafael,
> > >
> > > Yes, you are right!
> > > __driver_probe_device the only one use and just because
> > > __driver_probe_device use
> > > pm_runtime_get_suppliers cause problem.
> > >
> > >
> > > > Second, the role of pm_runtime_get_suppliers() is to "preactivate" the
> > > > suppliers before running probe for a consumer device and the role of
> > > the role of pm_runtime_get_suppliers() is to "preactivate" the suppliers,
> > > but suppliers may suspend immediately after preactivate right?
> > > Here is just this case. this is first racing point.
> > > Thread A: pm_runtime_get_suppliers                -> __driver_probe_device
> > > Thread B: pm_runtime_release_supplier
> > > Thread A: Run with supplier not preactivate      -> __driver_probe_device
> > >
> > > > pm_runtime_put_suppliers() is to do the cleanup in case the device is
> > > > left in suspend after probing.
> > > >
> > > > IOW, pm_runtime_get_suppliers() is to ensure that the suppliers will
> > > > be active until the probe callback takes over and the rest depends on
> > > > that callback.
> > > The problem of this racing will finally let consumer is active but
> > > supplier is suspended.
> > So it would be better to send a bug report regarding this.
> >
> > > The link relation is broken.
> > > I know you may curious how it happened? right?
> > > Honestly, I am not sure, but I think the second racing point
> > > is rpm_get_suppliers and pm_runtime_put_suppliers(release rpm_active).
> > I'm not sure what you mean by "the racing point".
> >
> > Yes, these functions can run concurrently.
> >
> > > So, I try to fix the first racing point and the problem is gone.
> > > It is full meet expect, and the pm runtime will work smoothly after
> > > __driver_probe_device done.
> > I'm almost sure that there is at least one scenario that would be
> > broken by this change.
> That said, the code in there may be a bit overdesigned.
>
> Does the patch below help?
>
> ---
>   drivers/base/power/runtime.c |   14 +-------------
>   1 file changed, 1 insertion(+), 13 deletions(-)
>
> Index: linux-pm/drivers/base/power/runtime.c
> ===================================================================
> --- linux-pm.orig/drivers/base/power/runtime.c
> +++ linux-pm/drivers/base/power/runtime.c
> @@ -1768,7 +1768,6 @@ void pm_runtime_get_suppliers(struct dev
>   		if (link->flags & DL_FLAG_PM_RUNTIME) {
>   			link->supplier_preactivated = true;
>   			pm_runtime_get_sync(link->supplier);
> -			refcount_inc(&link->rpm_active);
>   		}
>   
>   	device_links_read_unlock(idx);
> @@ -1788,19 +1787,8 @@ void pm_runtime_put_suppliers(struct dev
>   	list_for_each_entry_rcu(link, &dev->links.suppliers, c_node,
>   				device_links_read_lock_held())
>   		if (link->supplier_preactivated) {
> -			bool put;
> -
>   			link->supplier_preactivated = false;
> -
> -			spin_lock_irq(&dev->power.lock);
> -
> -			put = pm_runtime_status_suspended(dev) &&
> -			      refcount_dec_not_one(&link->rpm_active);
> -
> -			spin_unlock_irq(&dev->power.lock);
> -
> -			if (put)
> -				pm_runtime_put(link->supplier);
> +			pm_runtime_put(link->supplier);
>   		}
>   
>   	device_links_read_unlock(idx);


Hi Rafael,

I think this patch solve the rpm_active racing problem.
But it still have problem that
pm_runtime_get_suppliers call pm_runtime_get_sync(link->supplier)
and supplier could suspend immediately by other thread who call
pm_runtime_release_supplier.

Thanks.
Peter


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