ARM: 6279/1: highmem: fix SMP preemption bug in kmap_high_l1_vipt

smp_processor_id() must not be called from a preemptible context (this
is checked by CONFIG_DEBUG_PREEMPT).  kmap_high_l1_vipt() was doing so.
This lead to a problem where the wrong per_cpu kmap_high_l1_vipt_depth
could be incremented, causing a BUG_ON(*depth <= 0); in
kunmap_high_l1_vipt().

The solution is to move the call to smp_processor_id() after the call
to preempt_disable().

Originally by: Andrew Howe <ahowe@nvidia.com>

Signed-off-by: Gary King <gking@nvidia.com>
Acked-by: Nicolas Pitre <nico.as.pitre@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mm/highmem.c

@@ -163,19 +163,22 @@ static DEFINE_PER_CPU(int, kmap_high_l1_vipt_depth);
 
 void *kmap_high_l1_vipt(struct page *page, pte_t *saved_pte)
 {
-	unsigned int idx, cpu = smp_processor_id();
-	int *depth = &per_cpu(kmap_high_l1_vipt_depth, cpu);
+	unsigned int idx, cpu;
+	int *depth;
 	unsigned long vaddr, flags;
 	pte_t pte, *ptep;
 
+	if (!in_interrupt())
+		preempt_disable();
+
+	cpu = smp_processor_id();
+	depth = &per_cpu(kmap_high_l1_vipt_depth, cpu);
+
 	idx = KM_L1_CACHE + KM_TYPE_NR * cpu;
 	vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
 	ptep = TOP_PTE(vaddr);
 	pte = mk_pte(page, kmap_prot);
 
-	if (!in_interrupt())
-		preempt_disable();
-
 	raw_local_irq_save(flags);
 	(*depth)++;
 	if (pte_val(*ptep) == pte_val(pte)) {