10:37 a.m.
On 07/14/2017 05:23 AM, Erik Skultety wrote:
On Thu, Jul 13, 2017 at 07:23:58PM -0400, John Ferlan wrote:
>
>
> On 07/11/2017 10:38 AM, Erik Skultety wrote:
>> On Mon, Jul 03, 2017 at 05:25:30PM -0400, John Ferlan wrote:
>>> Since virnodedeviceobj now has a self-lockable hash table, there's no
>>> need to lock the table from the driver for processing. Thus remove the
>>> locks from the driver for NodeDeviceObjList mgmt.
>>>
>>> Signed-off-by: John Ferlan <jferlan(a)redhat.com>
>>
>> [..]
>>
>>> @@ -601,8 +565,6 @@ nodeDeviceDestroy(virNodeDevicePtr device)
>>> return -1;
>>> def = virNodeDeviceObjGetDef(obj);
>>>
>>> - nodeDeviceLock();
>>> -
>>
>> Consider the following scenario handling the same device:
>>
>
> What if I told you that's impossible? You cannot "have" a scsi_hostN,
> delete a scsi_hostN, and then have a new one created with the same name.
Except I wasn't considering creation, rather than plain change. Although I
didn't manage to find out under what circumstances would kernel actually emit
the 'change' event (I tried to write to various writable attributes - but since
it either lacks documentation completely or it's just well hidden from me - I
wasn't able to trigger it), other than explicitly triggering it by writing to
sysfs' uevent - so in that aspect, as I wrote below in my previous response,
it's highly unlikely but not impossible to hit the race.
Not quite sure what would trigger a change event. A vHBA has a wwnn/wwpn
and a parent wwnn/wwpn and WWN. If any of those change, the vHBA would
need to be deleted and recreated.
I do have a faint recollection of considering the ramifications of
dropping the obj lock in that path and the race drawback, but I
dismissed it mainly because of "how" vHBA's are created and what could
constitute a change event for a vHBA essentially redefines it.
>
> The scsi_hostN's are an ever increasing value of N. Once created and
> deleted, the N will not be reused.
>
>> Thread 1 (udev event handler callback) | Thread 2 (nodeDeviceDestroy)
>> =======================================|=============================
>> | # attempt to destroy a device
>> | obj = nodeDeviceObjFindByName()
>> |
>> | # @obj is locked
>> | def = virNodeDeviceObjGetDef(obj)
>> |
>> | virNodeDeviceObjEndAPI(&obj)
>> | # @obj is unlocked
>> <------
>> # change event |
>> udevAddOneDevice() |
>> |
>> obj = virNodeDeviceObjListFindByName |
>> # @obj locked |
>> new_device = false |
>> # @obj unlocked |
>> |
>> obj = virNodeDeviceObjListAssignDef |
>> # @obj locked |
>> virNodeDeviceObjEndAPI(&obj) |
>> # @obj unlocked and @def changed |
>> ------>
>> | virNodeDeviceObjListGetParentHost()
>> | if (def->parent) ===> SIGSEGV
>>
>> In patch 12/14 this would have been a deadlock because you first locked the
>> @obj, then nodedev driver while udevAddOneDevice did in the reverse order. I
>> don't know about NPIV so I'm not sure whether there is another way of
removing
>> a device and updating the parent device tree, might require an updated model,
>> but for now, you need to make a deep copy of @def. I can see that the chance of
>> getting an 'change' event from udev on a vHBA device is low, but I'm
trying to
>> look at it from a higher perspective, as we want to be able to remove mdevs
>> this way, possibly other devices in the future.
>
> I think what happens is code from virNodeDeviceGetWWNs through
> virVHBAManageVport gets placed into a function that handles vHBA's on
> deletion. Similarly for CreateXML, vHBA specific code ends up in a
> helper function. Those helpers would be called based on the type of
> object/device we're talking about (vHBA/mdev).
Despite the likelihood of the case I'm describing, the main point I'm trying to
make is that the lock protects a mutable resource (@def) and by releasing it
followed by querying it without actually holding the lock violates thread
safety.
I understand the position and while the likelihood is essentially next
to zero that something like that could happen it's also possible to
remove @def and pass copies of the name, parent, parent_wwnn,
parent_wwpn, and parent_fabric_wwn.
So before I do that - can we close on patches 1-12?
I can then post a v5 that alters the virNodeDeviceObjListGetParentHost
to take parameters instead of @def. That should allay this concern and
make patches 13/14 be 2 and 3 of the next series.
John
>
> BTW: I recall doing a review suggesting perhaps creating an mdev pool
> driver of sorts. Daniel essentially responded that using the node device
> driver and augmenting it to fit the needs would be OK. At the time, I
> wasn't specifically thinking about this case, but it certainly qualifies
> as something of concern where the existing node device code can make
> some assumptions about names that the underlying udev code provides.
>
> You may need to add some extra layer of protection if names can be
> reused especially because of this event mgmt "problem"... You may also
> need to modify that "if (dev)" code to check if the dev is an mdev and
> if so, do something different.
>
> Looks like the code was added by commit id '546fa3ef'. Perhaps as a way
> to "follow" how other drivers did things.
>
> "Fortunately" the only consumer of CreateXML/Destroy ends up being vHBA
> and we know there's rules around the naming. IIRC - you were letting
> MDEV also set the name, right? That is, a 'name' on input is essentially
No, mdev's name is a readonly attribute that is optional and exposed by the
vendor, the only thing libvirt can currently write to the mdev's sysfs
interface is UUID.
Erik
> and happily ignored. So what creates that name? And can you be assured
> it's going to be unique for the life/run time of the host? If so,
> there's no way a CreateXML could reuse a name that AddOneDevice would be
> using, right?
>
> Maybe I need to think some more - it's been a long day already
>
> John
>
>>
>> The rest of the patch looks okay, but I want to try it first.
>>
>> Erik
>>