powerpc/64s: Use dec_mm_active_cpus helper

Avoid open-coded atomic_dec on mm->context.active_cpus and use the
function made for it. Add CONFIG_DEBUG_VM underflow checking on the
counter.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230524060821.148015-3-npiggin@gmail.com

arch/powerpc/include/asm/book3s/64/mmu.h

@@ -261,7 +261,7 @@ static inline void radix_init_pseries(void) { }
 #define arch_clear_mm_cpumask_cpu(cpu, mm)				\
 	do {								\
 		if (cpumask_test_cpu(cpu, mm_cpumask(mm))) {		\
-			atomic_dec(&(mm)->context.active_cpus);		\
+			dec_mm_active_cpus(mm);				\
 			cpumask_clear_cpu(cpu, mm_cpumask(mm));		\
 		}							\
 	} while (0)

arch/powerpc/include/asm/mmu_context.h

@@ -127,6 +127,7 @@ static inline void inc_mm_active_cpus(struct mm_struct *mm)
 
 static inline void dec_mm_active_cpus(struct mm_struct *mm)
 {
+	VM_WARN_ON_ONCE(atomic_read(&mm->context.active_cpus) <= 0);
 	atomic_dec(&mm->context.active_cpus);
 }
 

arch/powerpc/mm/book3s64/radix_tlb.c

@@ -820,7 +820,7 @@ void exit_lazy_flush_tlb(struct mm_struct *mm, bool always_flush)
 	 * that's what the caller expects.
 	 */
 	if (cpumask_test_cpu(cpu, mm_cpumask(mm))) {
-		atomic_dec(&mm->context.active_cpus);
+		dec_mm_active_cpus(mm);
 		cpumask_clear_cpu(cpu, mm_cpumask(mm));
 		always_flush = true;
 	}