[PATCH] powerpc/rtas: Fix a potential race between CPU-Offline & Migration

[Nathan Fontenot]

On 09/27/2018 11:51 AM, Gautham R. Shenoy wrote:
> From: "Gautham R. Shenoy" <ego at linux.vnet.ibm.com>
> 
> Live Partition Migrations require all the present CPUs to execute the
> H_JOIN call, and hence rtas_ibm_suspend_me() onlines any offline CPUs
> before initiating the migration for this purpose.
> 
> The commit 85a88cabad57
> ("powerpc/pseries: Disable CPU hotplug across migrations")
> disables any CPU-hotplug operations once all the offline CPUs are
> brought online to prevent any further state change. Once the
> CPU-Hotplug operation is disabled, the code assumes that all the CPUs
> are online.
> 
> However, there is a minor window in rtas_ibm_suspend_me() between
> onlining the offline CPUs and disabling CPU-Hotplug when a concurrent
> CPU-offline operations initiated by the userspace can succeed thereby
> nullifying the the aformentioned assumption. In this unlikely case
> these offlined CPUs will not call H_JOIN, resulting in a system hang.
> 
> Fix this by verifying that all the present CPUs are actually online
> after CPU-Hotplug has been disabled, failing which we return from
> rtas_ibm_suspend_me() with -EBUSY.

Would we also want to havr the ability to re-try onlining all of the cpus
before failing the migration?

This would involve a bigger code change as the current code to online all
CPUs would work in its current form.

-Nathan

> 
> Cc: Nathan Fontenot <nfont at linux.vnet.ibm.com>
> Cc: Tyrel Datwyler <tyreld at linux.vnet.ibm.com>
> Suggested-by: Michael Ellerman <mpe at ellerman.id.au>
> Signed-off-by: Gautham R. Shenoy <ego at linux.vnet.ibm.com>
> ---
>  arch/powerpc/kernel/rtas.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
> index 2c7ed31..27f6fd3 100644
> --- a/arch/powerpc/kernel/rtas.c
> +++ b/arch/powerpc/kernel/rtas.c
> @@ -982,6 +982,16 @@ int rtas_ibm_suspend_me(u64 handle)
>  	}
> 
>  	cpu_hotplug_disable();
> +
> +	/* Check if we raced with a CPU-Offline Operation */
> +	if (unlikely(!cpumask_equal(cpu_present_mask, cpu_online_mask))) {
> +		pr_err("%s: Raced against a concurrent CPU-Offline\n",
> +		       __func__);
> +		atomic_set(&data.error, -EBUSY);
> +		cpu_hotplug_enable();
> +		goto out;
> +	}
> +
>  	stop_topology_update();
> 
>  	/* Call function on all CPUs.  One of us will make the
>