Re: [PATCH 3/4] pci aer: fix deadlock in do_recovery

From: Govindarajulu Varadarajan
Date: Sat Sep 30 2017 - 02:01:24 EST


On Fri, 29 Sep 2017, Sinan Kaya wrote:

On 9/28/2017 7:46 PM, Govindarajulu Varadarajan wrote:
How about releasing the device_lock here on CPU0?>

pci_device_add() is called by driver's pci probe function. device_lock(dev)
should be held before calling pci driver probe function.

I see. The goal of the lock held here is to protect probe() operation from
being disrupted. I also don't think we can change this.


or in other words keep device_lock as short as possible?

The problem is not the duration device_lock is held. It is the order two locks
are aquired. We cannot control or implement a restriction that during
device_lock() is held, driver probe should not call pci function which aquires
pci_bus_sem. And in case of pci aer, aer handler needs to call driver err_handler()
for which we need to hold device_lock() before calling err_handler(). In order
to find all the devices on a pci bus, we should hold pci_bus_sem to do
pci_walk_bus().

I was reacting to this to see if there is a better way to do this.

"Only fix I could think of is to lock &pci_bus_sem and try locking all
device->mutex under that pci_bus. If it fails, unlock all device->mutex
and &pci_bus_sem and try again."

How about gracefully returning from report_error_detected() when we cannot obtain
the device_lock() by replacing it with device_trylock()?


Some of the devices may miss the error reporthing. I have sent V2 where we do
a pci_bus_walk and adds all the devices to a list. After unlocking (up_read)
&pci_bus_sem, we go through the list and call err_handler of the devices with
devic_lock() held. This way, we dont try to hold both locks at same time and
we dont hold 50+ device_lock. Let me know if this approach is better.

aer_pci_walk_bus() can still poll like you did until it gets the lock. At least,
we don't get to introduce a new API, new lock semantics and code refactoring.
__pci_bus_trylock() looked very powerful and also dangerously flexible to
introduce new bugs to me.

For instance, you called it like this.

+ down_read(&pci_bus_sem);
+ locked = __pci_bus_trylock(bus, pci_device_trylock,
+ pci_device_unlock);

pci_bus_trylock() would obtain device + cfg locks whereas pci_device_trylock() only
obtains the device lock. Can it race against cfg lock? It depends on the caller.
Very subtle difference.

--
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.