Re: [RFC PATCH 1/6] Revert "PCI: Fix is_added/is_busmaster race condition"

[Lukas Wunner]

On Mon, Aug 20, 2018 at 12:10:59PM +1000, Benjamin Herrenschmidt wrote:
> I chose to create a new mutex which we should be able to address other
> similar races if we find them. The other solutions that I dismissed
> were:
> 
>  - Using the device_lock. A explained previously, this is tricky, I
> prefer not using this for anything other than locking against
> concurrent add/remove. The main issue is that drivers will be sometimes
> called in context where that's already held, so we can't take it inside
> pci_enable_device() and I'd rather not add new constraints such as
> "pci_enable_device() must be only called from probe() unless you also
> take the device lock". It would be tricky to audit everybody...
> 
>  - Using a global mutex. We could move the bridge lock from AER to core
> code for example, and use that. But it doesn't buy us much, and
> slightly redecuces parallelism. It also makes it a little bit more
> messy to walk up the bridge chain, we'd have to do a
> pci_enable_device_unlocked or something, messy.

+1 from my side for adding a struct mutex to struct pci_dev to protect
state changes.

The device_lock() primarily protects binding / unbinding of the device
and pci_dev state may have to be changed while binding / unbinding.

A global lock invites deadlocks if multiple devices are added / removed
concurrently where one is a parent of the other.  (Think hot-removal of
multiple devices on a Thunderbolt daisy-chain.)

As said I'd also welcome folding PCI_DEV_DISCONNECTED into enum
pci_channel_state, either as an additional state or by using
pci_channel_io_perm_failure.

Thanks,

Lukas