Re: [PATCH v6 03/13] vpci: move lock outside of struct vpci

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.02.2022 15:27, Roger Pau Monné wrote:
> On Mon, Feb 07, 2022 at 03:11:03PM +0100, Jan Beulich wrote:
>> On 07.02.2022 14:53, Oleksandr Andrushchenko wrote:
>>> On 07.02.22 14:46, Roger Pau Monné wrote:
>>>> I think the per-domain rwlock seems like a good option. I would do
>>>> that as a pre-patch.
>>> It is. But it seems it won't solve the thing we started this adventure for:
>>>
>>> With per-domain read lock and still ABBA in modify_bars (hope the below
>>> is correctly seen with a monospace font):
>>>
>>> cpu0: vpci_write-> d->RLock -> pdev1->lock ->                               
>>>                    rom_write -> modify_bars: tmp (pdev2) ->lock
>>> cpu1:        vpci_write-> d->RLock pdev2->lock -> cmd_write -> modify_bars: 
>>> tmp (pdev1) ->lock
>>>
>>> There is no API to upgrade read lock to write lock in modify_bars which 
>>> could help,
>>> so in both cases vpci_write should take write lock.
>>
>> Hmm, yes, I think you're right: It's not modify_bars() itself which needs
>> to acquire the write lock, but its (perhaps indirect) caller. Effectively
>> vpci_write() would need to take the write lock if the range written
>> overlaps the BARs or the command register.
> 
> I'm confused. If we use a per-domain rwlock approach there would be no
> need to lock tmp again in modify_bars, because we should hold the
> rwlock in write mode, so there's no ABBA?
> 
> We will have however to drop the per domain read and vpci locks and
> pick the per-domain lock in write mode.

Well, yes, with intermediate dropping of the lock acquiring in write mode
can be done in modify_bars(). I'm not convinced (yet) that such intermediate
dropping is actually going to be okay.

Jan