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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan