Re: [PATCH 2/2] driver core: Fix possible supplier PM-usage counter imbalance

[Jon Hunter <jonathanh@nvidia.com>]

On 15/02/2019 12:06, Rafael J. Wysocki wrote:
> On Friday, February 15, 2019 12:00:27 PM CET Jon Hunter wrote:
>> Hi Rafael,
>>
>> On 12/02/2019 12:08, Rafael J. Wysocki wrote:
>>> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>>>
>>> If a stateless device link to a certain supplier with
>>> DL_FLAG_PM_RUNTIME set in the flags is added and then removed by the
>>> consumer driver's probe callback, the supplier's PM-runtime usage
>>> counter will be nonzero after that which effectively causes the
>>> supplier to remain "always on" going forward.
>>>
>>> Namely, device_link_add() called to add the link invokes
>>> device_link_rpm_prepare() which notices that the consumer driver is
>>> probing, so it increments the supplier's PM-runtime usage counter
>>> with the assumption that the link will stay around until
>>> pm_runtime_put_suppliers() is called by driver_probe_device(),
>>> but if the link goes away before that point, the supplier's
>>> PM-runtime usage counter will remain nonzero.
>>>
>>> To prevent that from happening, first rework pm_runtime_get_suppliers()
>>> and pm_runtime_put_suppliers() to use the rpm_active refounts of device
>>> links and make the latter only drop rpm_active and the supplier's
>>> PM-runtime usage counter for each link by one, unless rpm_active is
>>> one already for it.  Next, modify device_link_add() to bump up the
>>> new link's rpm_active refcount and the suppliers PM-runtime usage
>>> counter by two, to prevent pm_runtime_put_suppliers(), if it is
>>> called subsequently, from suspending the supplier prematurely (in
>>> case its PM-runtime usage counter goes down to 0 in there).
>>>
>>> Due to the way rpm_put_suppliers() works, this change does not
>>> affect runtime suspend of the consumer ends of new device links (or,
>>> generally, device links for which DL_FLAG_PM_RUNTIME has just been
>>> set).
>>>
>>> Fixes: e2f3cd831a28 ("driver core: Fix handling of runtime PM flags in device_link_add()")
>>> Reported-by: Ulf Hansson <ulf.hansson@linaro.org> 
>>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>>> ---
>>>
>>> Note that the issue had been there before commit e2f3cd831a28, but it was
>>> overlooked by that commit and this change is a fix on top of it, so make
>>> the Fixes: tag point to commit e2f3cd831a28 (instead of an earlier one
>>> that the patch will not be applicable to).
>> I noticed that yesterday's and today's -next were no longer booting on
>> one of our Tegra boards (Tegra210 Jetson TX2) because networking is
>> failing. The ethernet chip is a USB device and looking at the bootlogs I
>> can see that the Tegra XHCI driver is failing ...
>>
>>  tegra-xusb 70090000.usb: xHCI host controller not responding, assume dead
>>  tegra-xusb 70090000.usb: HC died; cleaning up
>>
>> The Tegra XHCI driver uses multiple power-domains and uses
>> device_link_add() to attach them. So now I am wondering if there is
>> something that we have got wrong in our implementation. However, I don't
>> see the device being probed deferred on boot or anything like that.
>>
>> The driver in question is drivers/usb/host/xhci-tegra.c and we add the
>> links in the function tegra_xusb_powerdomain_init() which is before RPM
>> is enabled. Let me know if you have any thoughts.
> 
> Please try the appended patch on top of the $subject one (provided that
> reverting the $subject patch makes the problem go away).

Thanks and yes to confirm, reverting the $subject patch on top of next
does make the issue go away.

> ---
>  drivers/base/power/runtime.c |    9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> Index: linux-pm/drivers/base/power/runtime.c
> ===================================================================
> --- linux-pm.orig/drivers/base/power/runtime.c
> +++ linux-pm/drivers/base/power/runtime.c
> @@ -1675,9 +1675,12 @@ void pm_runtime_put_suppliers(struct dev
>  	idx = device_links_read_lock();
>  
>  	list_for_each_entry_rcu(link, &dev->links.suppliers, c_node)
> -		if (link->flags & DL_FLAG_PM_RUNTIME &&
> -		    refcount_dec_not_one(&link->rpm_active))
> -			pm_runtime_put(link->supplier);
> +		if (link->flags & DL_FLAG_PM_RUNTIME) {
> +			if (refcount_dec_not_one(&link->rpm_active))
> +				pm_runtime_put(link->supplier);
> +			else
> +				pm_request_idle(link->supplier);
> +		}
>  
>  	device_links_read_unlock(idx);
>  }

I will try this now and report back in a bit.

Cheers
Jon

-- 
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