[PATCH v3 2/3] hvc_init(): Enforce one-time initialization.

[Miche Baker-Harvey]

So on a CONSOLE_PORT_ADD message, we would take the
(existing)ports_device::ports_lock, and for other control messages we
would justtake the (new) port::port_lock?  You are concerned that just
takingthe ports_lock for all control messages could be too
restrictive?  Iwouldn't have expected these messages to be frequent
occurrences, butI'll defer to your experience here.
The CONSOLE_CONSOLE_PORT message calls hvc_alloc, which also
needsserialization.  That's in another one of these three patches; are
youthinking we could leave that patch be, or that we would we use
theport_lock for CONSOLE_CONSOLE_PORT?  Using the port_lock
wouldprovide the HVC serialization "for free" but it would be cleaner
if weput HVC related synchronization in hvc_console.c.
On Thu, Dec 8, 2011 at 4:08 AM, Amit Shah <amit.shah at redhat.com> wrote:
> On (Tue) 06 Dec 2011 [09:05:38], Miche Baker-Harvey wrote:
>> Amit,
>>
>> Ah, indeed.  I am not using MSI-X, so virtio_pci::vp_try_to_find_vqs()
>> calls vp_request_intx() and sets up an interrupt callback.  From
>> there, when an interrupt occurs, the stack looks something like this:
>>
>> virtio_pci::vp_interrupt()
>>   virtio_pci::vp_vring_interrupt()
>>     virtio_ring::vring_interrupt()
>>       vq->vq.callback()  <-- in this case, that's virtio_console::control_intr()
>>         workqueue::schedule_work()
>>           workqueue::queue_work()
>>             queue_work_on(get_cpu())  <-- queues the work on the current CPU.
>>
>> I'm not doing anything to keep multiple control message from being
>> sent concurrently to the guest, and we will take those interrupts on
>> any CPU. I've confirmed that the two instances of
>> handle_control_message() are occurring on different CPUs.
>
> So let's have a new helper, port_lock() that takes the port-specific
> spinlock.  There has to be a new helper, since the port lock should
> depend on the portdev lock being taken too.  For the port addition
> case, just the portdev lock should be taken.  For any other
> operations, the port lock should be taken.
>
> My assumption was that we would be able to serialise the work items,
> but that will be too restrictive.  Taking port locks sounds like a
> better idea.
>
> We'd definitely need the port lock in the control work handler.  We
> might need it in a few more places (like module removal), but we'll
> worry about that later.
>
> Does this sound fine?
>
>                Amit