scftorture: Prevent compiler from reducing race probabilities

Detecting smp_call_function() memory misordering requires close timing,
so it is necessary to have the checks immediately before and after
the call to the smp_call_function*() function under test.  This commit
therefore inserts barrier() calls to prevent the compiler from optimizing
memory-misordering detection down into the zone of extreme improbability.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/scftorture.c

@@ -322,6 +322,7 @@ static void scftorture_invoke_one(struct scf_statistics *scfp, struct torture_ra
 			scfp->n_single++;
 		if (scfcp) {
 			scfcp->scfc_cpu = cpu;
+			barrier(); // Prevent race-reduction compiler optimizations.
 			scfcp->scfc_in = true;
 		}
 		ret = smp_call_function_single(cpu, scf_handler_1, (void *)scfcp, scfsp->scfs_wait);
@@ -339,8 +340,10 @@ static void scftorture_invoke_one(struct scf_statistics *scfp, struct torture_ra
 			scfp->n_many_wait++;
 		else
 			scfp->n_many++;
-		if (scfcp)
+		if (scfcp) {
+			barrier(); // Prevent race-reduction compiler optimizations.
 			scfcp->scfc_in = true;
+		}
 		smp_call_function_many(cpu_online_mask, scf_handler, scfcp, scfsp->scfs_wait);
 		break;
 	case SCF_PRIM_ALL:
@@ -348,8 +351,10 @@ static void scftorture_invoke_one(struct scf_statistics *scfp, struct torture_ra
 			scfp->n_all_wait++;
 		else
 			scfp->n_all++;
-		if (scfcp)
+		if (scfcp) {
+			barrier(); // Prevent race-reduction compiler optimizations.
 			scfcp->scfc_in = true;
+		}
 		smp_call_function(scf_handler, scfcp, scfsp->scfs_wait);
 		break;
 	}
@@ -358,6 +363,7 @@ static void scftorture_invoke_one(struct scf_statistics *scfp, struct torture_ra
 			atomic_inc(&n_mb_out_errs); // Leak rather than trash!
 		else
 			kfree(scfcp);
+		barrier(); // Prevent race-reduction compiler optimizations.
 	}
 	if (use_cpus_read_lock)
 		cpus_read_unlock();