Re: [RFC PATCH 00/10] Rework PCI locking

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.01.2023 19:21, Julien Grall wrote:
> On 06/09/2022 11:32, Jan Beulich wrote:
>> On 31.08.2022 16:10, Volodymyr Babchuk wrote:
>>> Hello,
>>>
>>> This is yet another take to a PCI locking rework. This approach
>>> was suggest by Jan Beulich who proposed to use a reference
>>> counter to control lifetime of pci_dev objects.
>>>
>>> When I started added reference counting it quickly became clear
>>> that this approach can provide more granular locking insted of
>>> huge pcidevs_lock() which is used right now. I studied how this
>>> lock used and what it protects. And found the following:
>>>
>>> 0. Comment in pci.h states the following:
>>>
>>>   153 /*
>>>   154  * The pcidevs_lock protect alldevs_list, and the assignment for the
>>>   155  * devices, it also sync the access to the msi capability that is not
>>>   156  * interrupt handling related (the mask bit register).
>>>   157  */
>>>
>>> But in reality it does much more. Here is what I found:
>>>
>>> 1. Lifetime of pci_dev struct
>>>
>>> 2. Access to pseg->alldevs_list
>>>
>>> 3. Access to domain->pdev_list
>>>
>>> 4. Access to iommu->ats_list
>>>
>>> 5. Access to MSI capability
>>>
>>> 6. Some obsucure stuff in IOMMU drivers: there are places that
>>> are guarded by pcidevs_lock() but it seems that nothing
>>> PCI-related happens there.
>>
>> Right - the lock being held was (ab)used in IOMMU code in a number of
>> places. This likely needs to change in the course of this re-work;
>> patch titles don't suggest this is currently part of the series.
>>
>>> 7. Something that I probably overlooked
>>
>> And this is the main risk here. The huge scope of the original lock
>> means that many things are serialized now but won't be anymore once
>> the lock is gone.
>>
>> But yes - thanks for the work. To be honest I don't expect to be able
>> to look at this series in detail until after the Xen Summit. And even
>> then it may take a while ...
> 
> I was wondering if this is still in your list to review?

Yes, it certainly is. But as before no predictions when I might get to it.

Jan