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

[KY Srinivasan]

> -----Original Message-----
> From: Dexuan Cui
> Sent: Wednesday, August 5, 2015 9:44 PM
> To: David Miller <davem@xxxxxxxxxxxxx>; KY Srinivasan
> <kys@xxxxxxxxxxxxx>
> Cc: olaf@xxxxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx; jasowang@xxxxxxxxxx;
> driverdev-devel@xxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> stephen@xxxxxxxxxxxxxxxxxx; stefanha@xxxxxxxxxx;
> netdev@xxxxxxxxxxxxxxx; apw@xxxxxxxxxxxxx; pebolle@xxxxxxxxxx;
> dan.carpenter@xxxxxxxxxx
> Subject: RE: [PATCH V4 7/7] Drivers: hv: vmbus: disable local interrupt when
> hvsock's callback is running
> 
> > -----Original Message-----
> > From: devel [mailto:driverdev-devel-bounces@xxxxxxxxxxxxxxxxxxxxxx] On
> Behalf
> > Of Dexuan Cui
> > Sent: Thursday, July 30, 2015 18:18
> > To: David Miller <davem@xxxxxxxxxxxxx>; KY Srinivasan
> <kys@xxxxxxxxxxxxx>
> > Cc: olaf@xxxxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx; jasowang@xxxxxxxxxx;
> > driverdev-devel@xxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > stephen@xxxxxxxxxxxxxxxxxx; stefanha@xxxxxxxxxx;
> netdev@xxxxxxxxxxxxxxx;
> > apw@xxxxxxxxxxxxx; pebolle@xxxxxxxxxx; dan.carpenter@xxxxxxxxxx
> > Subject: RE: [PATCH V4 7/7] Drivers: hv: vmbus: disable local interrupt
> when
> > hvsock's callback is running
> >
> > > From: David Miller
> > > Sent: Thursday, July 30, 2015 6:28
> > > > From: Dexuan Cui <decui@xxxxxxxxxxxxx>
> > > > 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.
> >
> > -- Dexuan
> 
> Hi David, KY and all,
> 
> May I know your opinion about my idea of adding tasklet_disable/enbable()
> in vmbus_close_internal() and dropping this patch?

Sorry for the delayed response; I think this is a reasonable solution. Send me the patch.

Regards,

K. Y
> 
> Thanks,
> -- Dexuan

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