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: NJeremy Linton <jeremy.linton@arm.com>
Signed-off-by: NDavidlohr Bueso <dbueso@suse.de>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJeremy Linton <jeremy.linton@arm.com>
Tested-by: NJeremy 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();
 
@@ -909,7 +915,7 @@ 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) {
@@ -917,10 +923,12 @@ static int __init lock_torture_init(void)
 			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;
 		}
+	}
 
 	if (cxt.cur_ops->readlock) {
 		if (nreaders_stress >= 0)
@@ -936,6 +944,7 @@ static int __init lock_torture_init(void)
 			cxt.nrealreaders_stress = cxt.nrealwriters_stress;
 		}
 
+		if (nreaders_stress) {
 			lock_is_read_held = 0;
 			cxt.lrsa = kmalloc(sizeof(*cxt.lrsa) * cxt.nrealreaders_stress, GFP_KERNEL);
 			if (cxt.lrsa == NULL) {
@@ -951,6 +960,7 @@ static int __init lock_torture_init(void)
 				cxt.lrsa[i].n_lock_acquired = 0;
 			}
 		}
+	}
 
 	lock_torture_print_module_parms(cxt.cur_ops, "Start of test");
 
@@ -978,6 +988,7 @@ static int __init lock_torture_init(void)
 			goto unwind;
 	}
 
+	if (nwriters_stress) {
 		writer_tasks = kzalloc(cxt.nrealwriters_stress * sizeof(writer_tasks[0]),
 				       GFP_KERNEL);
 		if (writer_tasks == NULL) {
@@ -985,6 +996,7 @@ static int __init lock_torture_init(void)
 			firsterr = -ENOMEM;
 			goto unwind;
 		}
+	}
 
 	if (cxt.cur_ops->readlock) {
 		reader_tasks = kzalloc(cxt.nrealreaders_stress * sizeof(reader_tasks[0]),