[v2] powerpc/rtas: Fix a potential race between CPU-Offline & Migration

[Michael Ellerman]

On Mon, 2018-10-01 at 10:40:39 UTC, "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 restore the
> state of the offline CPUs in rtas_ibm_suspend_me() and return an
> -EBUSY.
> 
> 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>
> Reviewed-by: Nathan Fontenot <nfont at linux.vnet.ibm.com>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/dfd718a2ed1f678e66749ffe41bdea

cheers