powerpc: Make doorbell check preemption safe

Doorbell can be used to cause ipi on cpus which are sibling threads on
the same core. So icp_native_cause_ipi checks if the destination cpu
is a sibling thread of the current cpu and uses doorbell in such cases.

But while running with CONFIG_PREEMPT=y, since this section is
preemtible, we can run into issues if after we check if the destination
cpu is a sibling cpu, the task gets migrated from a sibling cpu to a
cpu on another core.

Fix this by using get_cpu()/ put_cpu()
Signed-off-by: Shreyas B. Prabhu <shreyas@linux.vnet.ibm.com>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/sysdev/xics/icp-native.c

@@ -147,10 +147,14 @@ static void icp_native_cause_ipi(int cpu, unsigned long data)
 {
 	kvmppc_set_host_ipi(cpu, 1);
 #ifdef CONFIG_PPC_DOORBELL
-	if (cpu_has_feature(CPU_FTR_DBELL) &&
-	    (cpumask_test_cpu(cpu, cpu_sibling_mask(smp_processor_id()))))
+	if (cpu_has_feature(CPU_FTR_DBELL)) {
+		if (cpumask_test_cpu(cpu, cpu_sibling_mask(get_cpu()))) {
 			doorbell_cause_ipi(cpu, data);
-	else
+			put_cpu();
+			return;
+		}
+		put_cpu();
+	}
 #endif
 	icp_native_set_qirr(cpu, IPI_PRIORITY);
 }