x86/resctrl: Use task_curr() instead of task_struct->on_cpu to prevent unnecessary IPI

James reported in [1] that there could be two tasks running on the same CPU
with task_struct->on_cpu set. Using task_struct->on_cpu as a test if a task
is running on a CPU may thus match the old task for a CPU while the
scheduler is running and IPI it unnecessarily.

task_curr() is the correct helper to use. While doing so move the #ifdef
check of the CONFIG_SMP symbol to be a C conditional used to determine
if this helper should be used to ensure the code is always checked for
correctness by the compiler.

[1] https://lore.kernel.org/lkml/a782d2f3-d2f6-795f-f4b1-9462205fd581@arm.comReported-by: James Morse <james.morse@arm.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/e9e68ce1441a73401e08b641cc3b9a3cf13fe6d4.1608243147.git.reinette.chatre@intel.com

arch/x86/kernel/cpu/resctrl/rdtgroup.c

@@ -2313,19 +2313,15 @@ static void rdt_move_group_tasks(struct rdtgroup *from, struct rdtgroup *to,
 			t->closid = to->closid;
 			t->rmid = to->mon.rmid;
 
-#ifdef CONFIG_SMP
 			/*
-			 * This is safe on x86 w/o barriers as the ordering
-			 * of writing to task_cpu() and t->on_cpu is
-			 * reverse to the reading here. The detection is
-			 * inaccurate as tasks might move or schedule
-			 * before the smp function call takes place. In
-			 * such a case the function call is pointless, but
+			 * If the task is on a CPU, set the CPU in the mask.
+			 * The detection is inaccurate as tasks might move or
+			 * schedule before the smp function call takes place.
+			 * In such a case the function call is pointless, but
 			 * there is no other side effect.
 			 */
-			if (mask && t->on_cpu)
+			if (IS_ENABLED(CONFIG_SMP) && mask && task_curr(t))
 				cpumask_set_cpu(task_cpu(t), mask);
-#endif
 		}
 	}
 	read_unlock(&tasklist_lock);