sched/membarrier: fix NULL poiner in membarrier_global_expedited

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I3UKOW


CVE: NA

--------------------------------

In commit 150593bf ("sched/api: Introduce task_rcu_dereference() and
try_get_task_struct()"), task_rcu_dereference() returns NULL if the task
go away before rcu_read_unlock().

this cause NULL pointer in membarrier_global_expedited(), fix it.

Fixes: 08946ecc ("sched/membarrier: Fix p->mm->membarrier_state racy load")
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>

kernel/sched/membarrier.c

@@ -115,7 +115,7 @@ static int membarrier_global_expedited(void)
 		 * scheduling a kthread.
 		 */
 		p = task_rcu_dereference(&cpu_rq(cpu)->curr);
-		if (p->flags & PF_KTHREAD)
+		if (p && p->flags & PF_KTHREAD)
 			continue;
 
 		__cpumask_set_cpu(cpu, tmpmask);