x86, UV: Fix smp_processor_id() use in a preemptable region

Fix a call by tunables_write() to smp_processor_id() within a
preemptable region.

Call get_cpu()/put_cpu() around the region where the returned
cpu number is actually used, which makes it non-preemptable.

A DEBUG_PREEMPT warning is prevented.

UV does not support cpu hotplug yet, but this is a step toward
that ability as well.

Signed-off-by: Cliff Wickman <cpw@sgi.com>
Link: http://lkml.kernel.org/r/20110621122242.086384966@sgi.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/platform/uv/tlb_uv.c

@@ -1334,9 +1334,10 @@ static ssize_t tunables_write(struct file *file, const char __user *user,
 
 	instr[count] = '\0';
 
-	bcp = &per_cpu(bau_control, smp_processor_id());
-
+	cpu = get_cpu();
+	bcp = &per_cpu(bau_control, cpu);
 	ret = parse_tunables_write(bcp, instr, count);
+	put_cpu();
 	if (ret)
 		return ret;