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

[Jan Beulich <jbeulich@suse.com>]

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.

Indeed.

Jan