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

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Date:	Mon, 12 Dec 2011 11:11:55 -0800
From:	Miche Baker-Harvey <miche@...gle.com>
To:	Amit Shah <amit.shah@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	xen-devel@...ts.xensource.com,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Anton Blanchard <anton@...ba.org>,
	Mike Waychison <mikew@...gle.com>,
	ppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	Eric Northrup <digitaleric@...gle.com>
Subject: Re: [PATCH v3 2/3] hvc_init(): Enforce one-time initialization.

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@...hat.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
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