Subject: Re: [PATCH 6/7] PCI: Make sure VF's driver get attached after PF's

From: Alexander Duyck <[email-masked]>
Date: Tue, 21 May 2013 14:58:58 -0700

On 05/21/2013 02:31 PM, Don Dutile wrote:
> On 05/21/2013 05:30 PM, Don Dutile wrote:
>> On 05/14/2013 05:39 PM, Alexander Duyck wrote:
>>> On 05/14/2013 12:59 PM, Yinghai Lu wrote:
>>>> On Tue, May 14, 2013 at 12:45 PM, Alexander Duyck
>>>> <[email-masked]> wrote:
>>>>> On 05/14/2013 11:44 AM, Yinghai Lu wrote:
>>>>>> On Tue, May 14, 2013 at 9:00 AM, Alexander Duyck
>>>>>> <[email-masked]> wrote:
>>>>>>> I'm sorry, but what is the point of this patch? With device
>>>>>>> assignment
>>>>>>> it is always possible to have VFs loaded and the PF driver unloaded
>>>>>>> since you cannot remove the VFs if they are assigned to a VM.
>>>>>> unload PF driver will not call pci_disable_sriov?
>>>>> You cannot call pci_disable_sriov because you will panic all of the
>>>>> guests that have devices assigned.
>>>> ixgbe_remove did call pci_disable_sriov...
>>>>
>>>> for guest panic, that is another problem.
>>>> just like you pci passthrough with real pci device and hotremove the
>>>> card in host.
>>>>
>>>> ...
>>>
>>> I suggest you take another look. In ixgbe_disable_sriov, which is the
>>> function that is called we do a check for assigned VFs. If they are
>>> assigned then we do not call pci_disable_sriov.
>>>
>>>>
>>>>> So how does your patch actually fix this problem? It seems like it is
>>>>> just avoiding it.
>>>> yes, until the first one is done.
>>>
>>> Avoiding the issue doesn't fix the underlying problem and instead you
>>> are likely just introducing more bugs as a result.
>>>
>>>>> From what I can tell your problem is originating in pci_call_probe. I
>>>>> believe it is calling work_on_cpu and that doesn't seem correct since
>>>>> the work should be taking place on a CPU already local to the PF. You
>>>>> might want to look there to see why you are trying to schedule work
>>>>> on a
>>>>> CPU which should be perfectly fine for you to already be doing your
>>>>> work on.
>>>> it always try to go with local cpu with same pxm.
>>>
>>> The problem is we really shouldn't be calling work_for_cpu in this case
>>> since we are already on the correct CPU. What probably should be
>>> happening is that pci_call_probe should be doing a check to see if the
>>> current CPU is already contained within the device node map and if so
>>> just call local_pci_probe directly. That way you can avoid deadlocking
>>> the system by trying to flush the CPU queue of the CPU you are
>>> currently on.
>>>
>> That's the patch that Michael Tsirkin posted for a fix,
>> but it was noted that if you have the case where the _same_ driver is
>> used for vf & pf,
>> other deadlocks may occur.
>> It would work in the case of ixgbe/ixgbevf, but not for something like
>> the Mellanox pf/vf driver (which is the same).
>>
> apologies; here's the thread the discussed the issue:
> https://patchwork.kernel.org/patch/2458681/
> 

I found out about that patch after I submitted one that was similar.
The only real complaint I had about his patch was that it was only
looking at the CPU and he could save himself some trouble by just doing
the work locally if we were on the correct NUMA node.  For example if
the system only has one node in it what is the point in scheduling all
of the work on CPU 0?  My alternative patch can be found at:
https://patchwork.kernel.org/patch/2568881/

As far as the inter-driver locking issues for same driver I don't think
that is really any kind if issue.  Most drivers shouldn't be holding any
big locks when they call pci_enable_sriov.  If I am not mistaken the
follow on patch I submitted which was similar to Michaels was reported
to have resolved the issue.

As far as the Mellanox PF/VF the bigger issue is that when they call
pci_enable_sriov they are not ready to handle VFs.  There have been
several suggestions on how to resolve it including -EPROBE_DEFER or the
igbvf/ixgbevf approach of just brining up the device in a "link down" state.

Thanks,

Alex