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

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>]

On 04.02.22 14:47, Jan Beulich wrote:
> On 04.02.2022 13:37, Oleksandr Andrushchenko wrote:
>>
>> On 04.02.22 13:37, Jan Beulich wrote:
>>> On 04.02.2022 12:13, Roger Pau Monné wrote:
>>>> On Fri, Feb 04, 2022 at 11:49:18AM +0100, Jan Beulich wrote:
>>>>> On 04.02.2022 11:12, Oleksandr Andrushchenko wrote:
>>>>>> On 04.02.22 11:15, Jan Beulich wrote:
>>>>>>> On 04.02.2022 09:58, Oleksandr Andrushchenko wrote:
>>>>>>>> On 04.02.22 09:52, Jan Beulich wrote:
>>>>>>>>> On 04.02.2022 07:34, Oleksandr Andrushchenko wrote:
>>>>>>>>>> @@ -285,6 +286,12 @@ static int modify_bars(const struct pci_dev 
>>>>>>>>>> *pdev, uint16_t cmd, bool rom_only)
>>>>>>>>>>                      continue;
>>>>>>>>>>              }
>>>>>>>>>>      
>>>>>>>>>> +        spin_lock(&tmp->vpci_lock);
>>>>>>>>>> +        if ( !tmp->vpci )
>>>>>>>>>> +        {
>>>>>>>>>> +            spin_unlock(&tmp->vpci_lock);
>>>>>>>>>> +            continue;
>>>>>>>>>> +        }
>>>>>>>>>>              for ( i = 0; i < ARRAY_SIZE(tmp->vpci->header.bars); 
>>>>>>>>>> i++ )
>>>>>>>>>>              {
>>>>>>>>>>                  const struct vpci_bar *bar = 
>>>>>>>>>> &tmp->vpci->header.bars[i];
>>>>>>>>>> @@ -303,12 +310,14 @@ static int modify_bars(const struct pci_dev 
>>>>>>>>>> *pdev, uint16_t cmd, bool rom_only)
>>>>>>>>>>                  rc = rangeset_remove_range(mem, start, end);
>>>>>>>>>>                  if ( rc )
>>>>>>>>>>                  {
>>>>>>>>>> +                spin_unlock(&tmp->vpci_lock);
>>>>>>>>>>                      printk(XENLOG_G_WARNING "Failed to remove [%lx, 
>>>>>>>>>> %lx]: %d\n",
>>>>>>>>>>                             start, end, rc);
>>>>>>>>>>                      rangeset_destroy(mem);
>>>>>>>>>>                      return rc;
>>>>>>>>>>                  }
>>>>>>>>>>              }
>>>>>>>>>> +        spin_unlock(&tmp->vpci_lock);
>>>>>>>>>>          }
>>>>>>>>> At the first glance this simply looks like another unjustified (in the
>>>>>>>>> description) change, as you're not converting anything here but you
>>>>>>>>> actually add locking (and I realize this was there before, so I'm 
>>>>>>>>> sorry
>>>>>>>>> for not pointing this out earlier).
>>>>>>>> Well, I thought that the description already has "...the lock can be
>>>>>>>> used (and in a few cases is used right away) to check whether vpci
>>>>>>>> is present" and this is enough for such uses as here.
>>>>>>>>>      But then I wonder whether you
>>>>>>>>> actually tested this, since I can't help getting the impression that
>>>>>>>>> you're introducing a live-lock: The function is called from 
>>>>>>>>> cmd_write()
>>>>>>>>> and rom_write(), which in turn are called out of vpci_write(). Yet 
>>>>>>>>> that
>>>>>>>>> function already holds the lock, and the lock is not (currently)
>>>>>>>>> recursive. (For the 3rd caller of the function - init_bars() - otoh
>>>>>>>>> the locking looks to be entirely unnecessary.)
>>>>>>>> Well, you are correct: if tmp != pdev then it is correct to acquire
>>>>>>>> the lock. But if tmp == pdev and rom_only == true
>>>>>>>> then we'll deadlock.
>>>>>>>>
>>>>>>>> It seems we need to have the locking conditional, e.g. only lock
>>>>>>>> if tmp != pdev
>>>>>>> Which will address the live-lock, but introduce ABBA deadlock potential
>>>>>>> between the two locks.
>>>>>> I am not sure I can suggest a better solution here
>>>>>> @Roger, @Jan, could you please help here?
>>>>> Well, first of all I'd like to mention that while it may have been okay to
>>>>> not hold pcidevs_lock here for Dom0, it surely needs acquiring when 
>>>>> dealing
>>>>> with DomU-s' lists of PCI devices. The requirement really applies to the
>>>>> other use of for_each_pdev() as well (in vpci_dump_msi()), except that
>>>>> there it probably wants to be a try-lock.
>>>>>
>>>>> Next I'd like to point out that here we have the still pending issue of
>>>>> how to deal with hidden devices, which Dom0 can access. See my RFC patch
>>>>> "vPCI: account for hidden devices in modify_bars()". Whatever the solution
>>>>> here, I think it wants to at least account for the extra need there.
>>>> Yes, sorry, I should take care of that.
>>>>
>>>>> Now it is quite clear that pcidevs_lock isn't going to help with avoiding
>>>>> the deadlock, as it's imo not an option at all to acquire that lock
>>>>> everywhere else you access ->vpci (or else the vpci lock itself would be
>>>>> pointless). But a per-domain auxiliary r/w lock may help: Other paths
>>>>> would acquire it in read mode, and here you'd acquire it in write mode (in
>>>>> the former case around the vpci lock, while in the latter case there may
>>>>> then not be any need to acquire the individual vpci locks at all). FTAOD:
>>>>> I haven't fully thought through all implications (and hence whether this 
>>>>> is
>>>>> viable in the first place); I expect you will, documenting what you've
>>>>> found in the resulting patch description. Of course the double lock
>>>>> acquire/release would then likely want hiding in helper functions.
>>>> I've been also thinking about this, and whether it's really worth to
>>>> have a per-device lock rather than a per-domain one that protects all
>>>> vpci regions of the devices assigned to the domain.
>>>>
>>>> The OS is likely to serialize accesses to the PCI config space anyway,
>>>> and the only place I could see a benefit of having per-device locks is
>>>> in the handling of MSI-X tables, as the handling of the mask bit is
>>>> likely very performance sensitive, so adding a per-domain lock there
>>>> could be a bottleneck.
>>> Hmm, with method 1 accesses serializing globally is basically
>>> unavoidable, but with MMCFG I see no reason why OSes may not (move
>>> to) permit(ting) parallel accesses, with serialization perhaps done
>>> only at device level. See our own pci_config_lock, which applies to
>>> only method 1 accesses; we don't look to be serializing MMCFG
>>> accesses at all.
>>>
>>>> We could alternatively do a per-domain rwlock for vpci and special case
>>>> the MSI-X area to also have a per-device specific lock. At which point
>>>> it becomes fairly similar to what you propose.
>> @Jan, @Roger
>>
>> 1. d->vpci_lock - rwlock <- this protects vpci
>> 2. pdev->vpci->msix_tbl_lock - rwlock <- this protects MSI-X tables
>> or should it better be pdev->msix_tbl_lock as MSI-X tables don't
>> really depend on vPCI?
> If so, perhaps indeed better the latter. But as said in reply to Roger,
> I'm not convinced (yet) that doing away with the per-device lock is a
> good move. As said there - we're ourselves doing fully parallel MMCFG
> accesses, so OSes ought to be fine to do so, too.
But with pdev->vpci_lock we face ABBA...
>
> Jan
>
>