Re: Kernel panic while doing vfio-pci hot-plug/unplug test

Xiang Zheng
2019-10-24T03:27:14Z

On 2019/10/24 4:46, Bjorn Helgaas wrote:
> [+cc Thomas, Rafael, beginning of thread at
> https://lore.kernel.org/r/79827f2f-9b43-4411-1376-b9063b67aee3@huawei.com]
>
> On Wed, Oct 23, 2019 at 09:38:51AM -0700, Matthew Wilcox wrote:
>> On Wed, Oct 23, 2019 at 10:15:40AM -0500, Bjorn Helgaas wrote:
>>> I don't like being one of a handful of callers of __add_wait_queue(),
>>> so I like that solution from that point of view.
>>>
>>> The 7ea7e98fd8d0 ("PCI: Block on access to temporarily unavailable pci
>>> device") commit log suggests that using __add_wait_queue() is a
>>> significant optimization, but I don't know how important that is in
>>> practical terms. Config accesses are never a performance path anyway,
>>> so I'd be inclined to use add_wait_queue() unless somebody complains.
>>
>> Wow, this has got pretty messy in the umpteen years since I last looked
>> at it.
>>
>> Some problems I see:
>>
>> 1. Commit df65c1bcd9b7b639177a5a15da1b8dc3bee4f5fa (tglx) says:
>>
>> x86/PCI: Select CONFIG_PCI_LOCKLESS_CONFIG
>>
>> All x86 PCI configuration space accessors have either their own
>> serialization or can operate completely lockless (ECAM).
>>
>> Disable the global lock in the generic PCI configuration space accessors.
>>
>> The concept behind this patch is broken. We still need to lock out
>> config space accesses when devices are undergoing D-state transitions.
>> I would suggest that for the contention case that tglx is concerned about,
>> we should have a pci_bus_read_config_unlocked_##size set of functions
>> which can be used for devices we know never go into D states.
>
> Host bridges that can't do config accesses atomically, e.g., they have
> something like the 0xcf8/0xcfc addr/data ports, need serialization.
> CONFIG_PCI_LOCKLESS_CONFIG removes the use of pci_lock for that, and I
> think that part makes sense regardless of whether devices can enter D
> states.
>
> We *should* prevent config accesses during D-state transitions (per
> PCIe r5.0, sec 5.9), but I don't think pci_lock ever did that.
> pci_raw_set_power_state() contains delays, but that only prevents
> accesses from the caller, not from other threads or from userspace.
> I suppose we should also prevent accesses by other threads during
> transitions done by ACPI, e.g., _PS0, _PS1, _PS2, _PS3. AFAICT we
> don't do any of that.
>
> It looks like pci_lock currently:
>
> - Serializes all kernel config accesses system-wide in
> pci_bus_read_config_##size() (unless CONFIG_PCI_LOCKLESS_CONFIG=y).
>
> - Serializes all userspace config accesses system-wide in
> pci_user_read_config_##size() (this seems unnecessary when
> CONFIG_PCI_LOCKLESS_CONFIG=y).
>
> - Serializes userspace config accesses with resets of the device via
> the dev->block_cfg_access bit and waitqueue mechanism.
>
> - Serializes kernel and userspace config accesses with bus->ops
> changes in pci_bus_set_ops() (except that we don't serialize
> kernel config accesses if CONFIG_PCI_LOCKLESS_CONFIG=y, which is
> probably a problem). But pci_bus_set_ops() is hardly used and I'm
> not sure it's worth keeping it.
>
>> 2. Commit a2e27787f893621c5a6b865acf6b7766f8671328 (jan kiszka)
>> exports pci_lock. I think this is a mistake; at best there should be
>> accessors for the pci_lock. But I don't understand why it needs to
>> exclude PCI config space changes throughout pci_check_and_set_intx_mask().
>> Why can it not do:
>>
>> - bus->ops->read(bus, dev->devfn, PCI_COMMAND, 4, &cmd_status_dword);
>> + pci_read_config_dword(dev, PCI_COMMAND, &cmd_status_dword);
>>
>> 3. I don't understand why 511dd98ce8cf6dc4f8f2cb32a8af31ce9f4ba4a1
>> changed pci_lock to be a raw spinlock. The patch description
>> essentially says "We need it for RT" which isn't terribly helpful.
>>
>> 4. Finally, getting back to the original problem report here, I wouldn't
>> write this code this way today. There's no reason not to use the
>> regular add_wait_queue etc. BUT! Why are we using this custom locking
>> mechanism? It pretty much screams to me of an rwsem (reads/writes
>> of config space take it for read; changes to config space accesses
>> (disabling and changing of accessor methods) take it for write.
>
> So maybe the immediate thing is to just convert to add_wait_queue()?

Hmmm... May I push a patch? :)

>
> There's a lot we could clean up here, but I think it would take a fair
> bit of untangling before we actually solve this panic.
>
> Bjorn
>
> .
>

--

Thanks,
Xiang