kgdb: use atomic_inc and atomic_dec instead of atomic_set

Memory barriers should be used for the kgdb cpu synchronization.  The
atomic_set() does not imply a memory barrier.
Reported-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NJason Wessel <jason.wessel@windriver.com>

kernel/kgdb.c

@@ -1379,8 +1379,7 @@ acquirelock:
 	 * Make sure the above info reaches the primary CPU before
 	 * our cpu_in_kgdb[] flag setting does:
 	 */
-	smp_wmb();
-	atomic_set(&cpu_in_kgdb[cpu], 1);
+	atomic_inc(&cpu_in_kgdb[cpu]);
 
 	/*
 	 * CPU will loop if it is a slave or request to become a kgdb
@@ -1400,7 +1399,7 @@ return_normal:
 			 */
 			if (arch_kgdb_ops.correct_hw_break)
 				arch_kgdb_ops.correct_hw_break();
-			atomic_set(&cpu_in_kgdb[cpu], 0);
+			atomic_dec(&cpu_in_kgdb[cpu]);
 			touch_softlockup_watchdog_sync();
 			clocksource_touch_watchdog();
 			local_irq_restore(flags);
@@ -1449,7 +1448,7 @@ return_normal:
 	 */
 	if (!kgdb_single_step) {
 		for (i = 0; i < NR_CPUS; i++)
-			atomic_set(&passive_cpu_wait[i], 1);
+			atomic_inc(&passive_cpu_wait[i]);
 	}
 
 #ifdef CONFIG_SMP
@@ -1483,11 +1482,11 @@ return_normal:
 	if (kgdb_io_ops->post_exception)
 		kgdb_io_ops->post_exception();
 
-	atomic_set(&cpu_in_kgdb[ks->cpu], 0);
+	atomic_dec(&cpu_in_kgdb[ks->cpu]);
 
 	if (!kgdb_single_step) {
 		for (i = NR_CPUS-1; i >= 0; i--)
-			atomic_set(&passive_cpu_wait[i], 0);
+			atomic_dec(&passive_cpu_wait[i]);
 		/*
 		 * Wait till all the CPUs have quit
 		 * from the debugger.
@@ -1736,11 +1735,11 @@ EXPORT_SYMBOL_GPL(kgdb_unregister_io_module);
  */
 void kgdb_breakpoint(void)
 {
-	atomic_set(&kgdb_setting_breakpoint, 1);
+	atomic_inc(&kgdb_setting_breakpoint);
 	wmb(); /* Sync point before breakpoint */
 	arch_kgdb_breakpoint();
 	wmb(); /* Sync point after breakpoint */
-	atomic_set(&kgdb_setting_breakpoint, 0);
+	atomic_dec(&kgdb_setting_breakpoint);
 }
 EXPORT_SYMBOL_GPL(kgdb_breakpoint);