[PATCH] avoid return NULL on root rb_node in rb_next/rb_prev in lib/rbtree.c

[shenghui]

Hi,

       I'm reading cfs code, and get the following potential bug.

In kernel/sched_fair.c, we can get the following call thread:

1778static struct task_struct *pick_next_task_fair(struct rq *rq)
1779{
...
1787        do {
1788                se = pick_next_entity(cfs_rq);
1789                set_next_entity(cfs_rq, se);
1790                cfs_rq = group_cfs_rq(se);
1791        } while (cfs_rq);
...
1797}

 925static struct sched_entity *pick_next_entity(struct cfs_rq *cfs_rq)
 926{
 927        struct sched_entity *se = __pick_next_entity(cfs_rq);
...
 941        return se;
 942}

 377static struct sched_entity *__pick_next_entity(struct cfs_rq *cfs_rq)
 378{
 379        struct rb_node *left = cfs_rq->rb_leftmost;
 380
 381        if (!left)
 382                return NULL;
 ...
 385}

To manipulate cfs_rq->rb_leftmost, __dequeue_entity does the following:

 365static void __dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 366{
 367        if (cfs_rq->rb_leftmost == &se->run_node) {
 368                struct rb_node *next_node;
 369
 370                next_node = rb_next(&se->run_node);
 371                cfs_rq->rb_leftmost = next_node;
 372        }
 373
 374        rb_erase(&se->run_node, &cfs_rq->tasks_timeline);
 375}

Here, if se->run_node is the root rb_node, next_node will be set NULL
by rb_next.
Then __pick_next_entity may get NULL on some call, and set_next_entity
may deference
NULL value.

 892static void
 893set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 894{
 895        /* 'current' is not kept within the tree. */
 896        if (se->on_rq) {
...
 919        se->prev_sum_exec_runtime = se->sum_exec_runtime;
 920}

Following is my patch. Please check it.