locking/locktorture: Fix num reader/writer corner cases

Things can explode for locktorture if the user does combinations
of nwriters_stress=0 nreaders_stress=0. Fix this by not assuming
we always want to torture writer threads.

Reported-by: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Jeremy Linton <jeremy.linton@arm.com>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>

kernel/locking/locktorture.c

@@ -703,8 +703,7 @@ static void __torture_print_stats(char *page,
 {
 	bool fail = 0;
 	int i, n_stress;
-	long max = 0;
-	long min = statp[0].n_lock_acquired;
+	long max = 0, min = statp ? statp[0].n_lock_acquired : 0;
 	long long sum = 0;
 
 	n_stress = write ? cxt.nrealwriters_stress : cxt.nrealreaders_stress;
@@ -811,7 +810,7 @@ static void lock_torture_cleanup(void)
 	 * such, only perform the underlying torture-specific cleanups,
 	 * and avoid anything related to locktorture.
 	 */
-	if (!cxt.lwsa)
+	if (!cxt.lwsa && !cxt.lrsa)
 		goto end;
 
 	if (writer_tasks) {
@@ -886,6 +885,13 @@ static int __init lock_torture_init(void)
 		firsterr = -EINVAL;
 		goto unwind;
 	}
+
+	if (nwriters_stress == 0 && nreaders_stress == 0) {
+		pr_alert("lock-torture: must run at least one locking thread\n");
+		firsterr = -EINVAL;
+		goto unwind;
+	}
+
 	if (cxt.cur_ops->init)
 		cxt.cur_ops->init();
 
@@ -909,17 +915,19 @@ static int __init lock_torture_init(void)
 #endif
 
 	/* Initialize the statistics so that each run gets its own numbers. */
+	if (nwriters_stress) {
+		lock_is_write_held = 0;
+		cxt.lwsa = kmalloc(sizeof(*cxt.lwsa) * cxt.nrealwriters_stress, GFP_KERNEL);
+		if (cxt.lwsa == NULL) {
+			VERBOSE_TOROUT_STRING("cxt.lwsa: Out of memory");
+			firsterr = -ENOMEM;
+			goto unwind;
+		}
 
-	lock_is_write_held = 0;
-	cxt.lwsa = kmalloc(sizeof(*cxt.lwsa) * cxt.nrealwriters_stress, GFP_KERNEL);
-	if (cxt.lwsa == NULL) {
-		VERBOSE_TOROUT_STRING("cxt.lwsa: Out of memory");
-		firsterr = -ENOMEM;
-		goto unwind;
-	}
-	for (i = 0; i < cxt.nrealwriters_stress; i++) {
-		cxt.lwsa[i].n_lock_fail = 0;
-		cxt.lwsa[i].n_lock_acquired = 0;
+		for (i = 0; i < cxt.nrealwriters_stress; i++) {
+			cxt.lwsa[i].n_lock_fail = 0;
+			cxt.lwsa[i].n_lock_acquired = 0;
+		}
 	}
 
 	if (cxt.cur_ops->readlock) {
@@ -936,19 +944,21 @@ static int __init lock_torture_init(void)
 			cxt.nrealreaders_stress = cxt.nrealwriters_stress;
 		}
 
-		lock_is_read_held = 0;
-		cxt.lrsa = kmalloc(sizeof(*cxt.lrsa) * cxt.nrealreaders_stress, GFP_KERNEL);
-		if (cxt.lrsa == NULL) {
-			VERBOSE_TOROUT_STRING("cxt.lrsa: Out of memory");
-			firsterr = -ENOMEM;
-			kfree(cxt.lwsa);
-			cxt.lwsa = NULL;
-			goto unwind;
-		}
-
-		for (i = 0; i < cxt.nrealreaders_stress; i++) {
-			cxt.lrsa[i].n_lock_fail = 0;
-			cxt.lrsa[i].n_lock_acquired = 0;
+		if (nreaders_stress) {
+			lock_is_read_held = 0;
+			cxt.lrsa = kmalloc(sizeof(*cxt.lrsa) * cxt.nrealreaders_stress, GFP_KERNEL);
+			if (cxt.lrsa == NULL) {
+				VERBOSE_TOROUT_STRING("cxt.lrsa: Out of memory");
+				firsterr = -ENOMEM;
+				kfree(cxt.lwsa);
+				cxt.lwsa = NULL;
+				goto unwind;
+			}
+
+			for (i = 0; i < cxt.nrealreaders_stress; i++) {
+				cxt.lrsa[i].n_lock_fail = 0;
+				cxt.lrsa[i].n_lock_acquired = 0;
+			}
 		}
 	}
 
@@ -978,12 +988,14 @@ static int __init lock_torture_init(void)
 			goto unwind;
 	}
 
-	writer_tasks = kzalloc(cxt.nrealwriters_stress * sizeof(writer_tasks[0]),
-			       GFP_KERNEL);
-	if (writer_tasks == NULL) {
-		VERBOSE_TOROUT_ERRSTRING("writer_tasks: Out of memory");
-		firsterr = -ENOMEM;
-		goto unwind;
+	if (nwriters_stress) {
+		writer_tasks = kzalloc(cxt.nrealwriters_stress * sizeof(writer_tasks[0]),
+				       GFP_KERNEL);
+		if (writer_tasks == NULL) {
+			VERBOSE_TOROUT_ERRSTRING("writer_tasks: Out of memory");
+			firsterr = -ENOMEM;
+			goto unwind;
+		}
 	}
 
 	if (cxt.cur_ops->readlock) {