On 09/27/2018 11:51 AM, Gautham R. Shenoy wrote:
> From: "Gautham R. Shenoy" <e...@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 <nf...@linux.vnet.ibm.com>
> Cc: Tyrel Datwyler <tyr...@linux.vnet.ibm.com>
> Suggested-by: Michael Ellerman <m...@ellerman.id.au>
> Signed-off-by: Gautham R. Shenoy <e...@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
>
