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

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Date:	Wed, 29 Jul 2015 15:28:09 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	decui@...rosoft.com
Cc:	gregkh@...uxfoundation.org, stephen@...workplumber.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	driverdev-devel@...uxdriverproject.org, olaf@...fle.de,
	apw@...onical.com, jasowang@...hat.com, kys@...rosoft.com,
	pebolle@...cali.nl, stefanha@...hat.com, vkuznets@...hat.com,
	dan.carpenter@...cle.com
Subject: Re: [PATCH V4 7/7] Drivers: hv: vmbus: disable local interrupt
 when hvsock's callback is running

From: Dexuan Cui <decui@...rosoft.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.
> 
> Signed-off-by: Dexuan Cui <decui@...rosoft.com>

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.
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