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

From: shenghui
Date: Mon Jun 28 2010 - 19:48:18 EST

2010/6/28 Peter Zijlstra <peterz@xxxxxxxxxxxxx>:
> So if ->rb_leftmost is NULL, then the if (!left) check in
> __pick_next_entity() would return null.
> As to the NULL deref in in pick_next_task_fair()->set_next_entity() that
> should never happen because pick_next_task_fair() will bail
> on !->nr_running.
> Furthermore, you've failed to mention what kernel version you're looking
> at.

The kernel version is 2.6.35-rc3, and 2.6.34 has the same code.

For nr->running, if current is the only process in the run queue, then
nr->running would not be zero.
1784 if (!cfs_rq->nr_running)
1785 return NULL;
pick_next_task_fair() could pass above check and run to following:
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);

Then pick_next_entity will get NULL for current is the root rb_node.
Then set_next_entity would fail on NULL deference.


Thanks and Best Regards,
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at