Messages in this thread | | | Subject | Re: [PATCH v1] PM-runtime: Check supplier_preactivated before release supplier | From | Peter Wang <> | Date | Thu, 30 Jun 2022 23:19:15 +0800 |
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On 6/30/22 10:47 PM, Rafael J. Wysocki wrote: > 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.
Hi Rafael,
I think pm_runtime_release_supplier will always decrese the reference rpm_active count to 1 and check idle will let supplier enter suspend. Am I wrong? Could you explain why this patch won't drop the reference?
Thanks
Peter
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