RE: [PATCH V4 7/7] Drivers: hv: vmbus: disable local interrupt when hvsock's callback is running

[Dexuan Cui <decui@microsoft.com>]

> From: David Miller
> Sent: Thursday, July 30, 2015 6:28
> > From: Dexuan Cui <decui@microsoft.com>
> > Date: Tue, 28 Jul 2015 05:35:30 -0700
> >
> > In the SMP guest case, when the per-channel callback hvsock_events() is
> > running on virtual CPU A, if the guest tries to close the connection on
> > virtual CPU B: we invoke vmbus_close() -> vmbus_close_internal(),
> > then we can have trouble: on B, vmbus_close_internal() will send IPI
> > reset_channel_cb() to A, trying to set channel->onchannel_callbackto NULL;
> > on A, if the IPI handler happens between
> > "if (channel->onchannel_callback != NULL)" and invoking
> > channel->onchannel_callback, we'll invoke a function pointer of NULL.
> >
> > This is why the patch is necessary.
> >
> Sorry, I do not accept that you must use conditional locking and/or
> IRQ disabling.
>
> Boil it down to what is necessary for the least common denominator,
> and use that unconditionally.

Hi David,
Thanks for the comment!

I agree with you it's not clean to use conditional IRQ disabling.

Here I didn't use unconditionally IRQ disabling because the Hyper-V netvsc
and storvsc driver's vmbus event callbacks (i.e. netvsc_channel_cb() and
storvsc_on_channel_callback()) may take relatively long time (e.g., netvsc can
operate at a speed of 10Gb) and I think it's bad to disable IRQ for long time
when the callbacks are running in a tasklet context, e.g., the Hyper-V timer
can be affected: see vmbus_isr() -> hv_process_timer_expiration().

To resolve the race condition between vmbus_close_internal() and
process_chn_event() in SMP case, now I propose a new method:

we can serialize the 2 paths by adding
tasklet_disable(hv_context.event_dpc[channel->target_cpu]) and
tasklet_enable(...) in vmbus_close_internal().

In this way, we need the least change and we can drop this patch.

Please let me know your opinion.

Thanks,
-- Dexuan