From: KY Srinivasan
Sent: Friday, August 7, 2015 1:50
To: Dexuan Cui <decui@microsoft.com>; David Miller <davem@davemloft.net>
Cc: olaf@aepfle.de; gregkh@linuxfoundation.org; jasowang@redhat.com;
driverdev-devel@linuxdriverproject.org; linux-kernel@vger.kernel.org;
stephen@networkplumber.org; stefanha@redhat.com; netdev@vger.kernel.org;
apw@canonical.com; pebolle@tiscali.nl; dan.carpenter@oracle.com
Subject: RE: [PATCH V4 7/7] Drivers: hv: vmbus: disable local interrupt when
hvsock's callback is running

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From: Dexuan Cui
Sent: Wednesday, August 5, 2015 9:44 PM
To: David Miller <davem@davemloft.net>; KY Srinivasan
[off-list ref]
Cc: olaf@aepfle.de; gregkh@linuxfoundation.org; jasowang@redhat.com;
driverdev-devel@linuxdriverproject.org; linux-kernel@vger.kernel.org;
stephen@networkplumber.org; stefanha@redhat.com;
netdev@vger.kernel.org; apw@canonical.com; pebolle@tiscali.nl;
dan.carpenter@oracle.com
Subject: RE: [PATCH V4 7/7] Drivers: hv: vmbus: disable local interrupt when
hvsock's callback is running

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From: devel [mailto:driverdev-devel-bounces@linuxdriverproject.org] On

Behalf

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Of Dexuan Cui
Sent: Thursday, July 30, 2015 18:18
To: David Miller <davem@davemloft.net>; KY Srinivasan

[off-list ref]

quoted

Cc: olaf@aepfle.de; gregkh@linuxfoundation.org; jasowang@redhat.com;
driverdev-devel@linuxdriverproject.org; linux-kernel@vger.kernel.org;
stephen@networkplumber.org; stefanha@redhat.com;

netdev@vger.kernel.org;

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apw@canonical.com; pebolle@tiscali.nl; dan.carpenter@oracle.com
Subject: RE: [PATCH V4 7/7] Drivers: hv: vmbus: disable local interrupt

when

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hvsock's callback is running

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From: David Miller
Sent: Thursday, July 30, 2015 6:28

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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

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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;

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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

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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

OK. Will do.

 Thanks,
 -- Dexuan