Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Thu, 30 Jun 2022 16:47:55 +0200 | Subject | Re: [PATCH v1] PM-runtime: Check supplier_preactivated before release supplier |
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On Thu, Jun 30, 2022 at 4:26 PM Peter Wang <peter.wang@mediatek.com> wrote: > > > 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@mediatek.com> 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@mediatek.com> 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@mediatek.com> wrote: > >>>>>>> On 6/28/22 3:00 AM, Rafael J. Wysocki wrote: > >>>>>>>> On Mon, Jun 13, 2022 at 2:08 PM <peter.wang@mediatek.com> wrote: > >>>>>>>>> From: Peter Wang <peter.wang@mediatek.com> > >>>>>>>>> > >>>>>>>>> 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.
No, it won't, because pm_runtime_release_supplier() won't drop the reference on the supplier taken by pm_runtime_get_suppliers(0 after the patch.
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