linux-kernel - Re: [PATCH] avoid return NULL on root rb_node in rb_next/rb

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Date:	Tue, 29 Jun 2010 07:48:12 +0800
From:	shenghui <crosslonelyover@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
	Greg KH <greg@...ah.com>, linux-mm@...ck.org, mingo@...e.hu
Subject: Re: [PATCH] avoid return NULL on root rb_node in rb_next/rb_prev in 
	lib/rbtree.c

2010/6/28 Peter Zijlstra <peterz@...radead.org>:
> 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,
shenghui
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