[PATCH] locktorture: Fix NULL pointer when torture_type is invalid

[Kefeng Wang <wangkefeng.wang@huawei.com>]

Insmod locktorture with torture_type=mutex will lead to crash,

Unable to handle kernel NULL pointer dereference at virtual address 00000008
pgd = ffffffc0f6c10000
[00000008] *pgd=000000013b221003, *pud=000000013b221003, *pmd=0000000000000000
Internal error: Oops: 94000006 [#1] PREEMPT SMP
Modules linked in: locktorture(+) torture
CPU: 2 PID: 1462 Comm: insmod Not tainted 4.4.0+ #19
Hardware name: linux,dummy-virt (DT)
task: ffffffc0fb2b3700 ti: ffffffc0fa938000 task.ti: ffffffc0fa938000
PC is at __torture_print_stats+0x18/0x180 [locktorture]
LR is at lock_torture_stats_print+0x68/0x110 [locktorture]
pc : [<ffffffbffc017028>] lr : [<ffffffbffc017500>] pstate: 60000145
sp : ffffffc0fa93bb20
[snip...]
Call trace:
[<ffffffbffc017028>] __torture_print_stats+0x18/0x180 [locktorture]
[<ffffffbffc017500>] lock_torture_stats_print+0x68/0x110 [locktorture]
[<ffffffbffc0180fc>] lock_torture_cleanup+0xc4/0x278 [locktorture]
[<ffffffbffc01d144>] lock_torture_init+0x144/0x5b0 [locktorture]
[<ffffffc000082940>] do_one_initcall+0x94/0x1a0
[<ffffffc000141888>] do_init_module+0x60/0x1c8
[<ffffffc00011c628>] load_module+0x1880/0x1c9c
[<ffffffc00011cc00>] SyS_finit_module+0x7c/0x88
[<ffffffc000085cb0>] el0_svc_naked+0x24/0x28

Fix it by check stats_task in lock_torture_cleanup() to guard not into
a wrong path.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
---
 kernel/locking/locktorture.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/locktorture.c b/kernel/locking/locktorture.c
index 8ef1919..1afa8ad 100644
--- a/kernel/locking/locktorture.c
+++ b/kernel/locking/locktorture.c
@@ -764,8 +764,10 @@ static void lock_torture_cleanup(void)
 		reader_tasks = NULL;
 	}
 
-	torture_stop_kthread(lock_torture_stats, stats_task);
-	lock_torture_stats_print();  /* -After- the stats thread is stopped! */
+	if (stats_task) {
+		torture_stop_kthread(lock_torture_stats, stats_task);
+		lock_torture_stats_print();  /* -After- the stats thread is stopped! */
+	}
 
 	if (atomic_read(&cxt.n_lock_torture_errors))
 		lock_torture_print_module_parms(cxt.cur_ops,
-- 
1.7.12.4