linux-kernel - Re: [PATCH 3.16.y-ckt 17/17] Drivers: hv: vmbus: prevent cpu offlining on newer hypervisors

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Message-ID: <1460661066.4560.19.camel@decadent.org.uk>
Date:	Thu, 14 Apr 2016 20:11:06 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Chas Williams <3chas3@...il.com>,
	Luis Henriques <luis.henriques@...onical.com>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Subject: Re: [PATCH 3.16.y-ckt 17/17] Drivers: hv: vmbus: prevent cpu
 offlining on newer hypervisors

On Tue, 2016-04-12 at 19:08 +0100, Luis Henriques wrote:
> 3.16.7-ckt27 -stable review patch.  If anyone has any objections, please let me know.
> 
> ---8<------------------------------------------------------------
> 
> From: Vitaly Kuznetsov <vkuznets@...hat.com>
> 
> commit e513229b4c386e6c9f66298c13fde92f73e6e1ac upstream.
> 
> When an SMP Hyper-V guest is running on top of 2012R2 Server and secondary
> cpus are sent offline (with echo 0 > /sys/devices/system/cpu/cpu$cpu/online)
> the system freeze is observed. This happens due to the fact that on newer
> hypervisors (Win8, WS2012R2, ...) vmbus channel handlers are distributed
> across all cpus (see init_vp_index() function in drivers/hv/channel_mgmt.c)
> and on cpu offlining nobody reassigns them to CPU0. Prevent cpu offlining
> when vmbus is loaded until the issue is fixed host-side.
> 
> This patch also disables hibernation but it is OK as it is also broken (MCE
> error is hit on resume). Suspend still works.
[...]
> +static void hv_cpu_hotplug_quirk(bool vmbus_loaded)
> +{
> +	static void *previous_cpu_disable;
> +
> +	/*
> +	 * Offlining a CPU when running on newer hypervisors (WS2012R2, Win8,
> +	 * ...) is not supported at this moment as channel interrupts are
> +	 * distributed across all of them.
> +	 */
> +
> +	if ((vmbus_proto_version == VERSION_WS2008) ||
> +	    (vmbus_proto_version == VERSION_WIN7))
> +		return;
> +
> +	if (vmbus_loaded) {
> +		previous_cpu_disable = smp_ops.cpu_disable;
> +		smp_ops.cpu_disable = hyperv_cpu_disable;
> +		pr_notice("CPU offlining is not supported by hypervisor\n");
> +	} else if (previous_cpu_disable)
> +		smp_ops.cpu_disable = previous_cpu_disable;
[...]

This is a really bad hack.  What if two different drivers patched
smp_ops and got unloaded in a different order?  Perhaps the core
support code for Hyper-V should define its own smp_ops.

I don't want to stop this going into stable, but seriously, please
clean this up.

Ben.

-- 
Ben Hutchings
In a hierarchy, every employee tends to rise to his level of incompetence.
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