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

[Govindarajulu Varadarajan]

On Thu, 28 Sep 2017, Sinan Kaya wrote:

> On 9/27/2017 5:42 PM, Govindarajulu Varadarajan wrote:
> > CPU0					CPU1
> > ---------------------------------------------------------------------
> > __driver_attach()
> > device_lock(&dev->mutex) <--- device mutex lock here
> > driver_probe_device()
> > pci_enable_sriov()
> > pci_iov_add_virtfn()
> > pci_device_add()
> > 					aer_isr()		<--- pci aer error
> > 					do_recovery()
> > 					broadcast_error_message()
> > 					pci_walk_bus()
> > 					down_read(&pci_bus_sem) <--- rd sem
>
> 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.

> 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().

> > down_write(&pci_bus_sem) <-- stuck on wr sem
> > 					report_error_detected()
> > 					device_lock(&dev->mutex)<--- DEAD LOCK