Re: [PATCH] sched: fix NULL pointer issue in pick_next_entity()
From: Peter Zijlstra
Date: Tue Aug 01 2017 - 04:38:22 EST
On Tue, Aug 01, 2017 at 06:01:56PM +0800, Yafang Shao wrote:
> When we select CFQ as the scheduler, in function pick_next_task_fair
CFS, CFQ is a block IO scheduler.
> it will pass NULL as the 2nd argument to pick_next_entity:
> pick_next_entity(cfs_rq, NULL);
> And once __pick_first_entity() is called, it could return NULL as well.
> So in function pick_next_entity(), the local variable 'left' and 'curr'
> could both be NULL, then this will cause NULL pointer issue.
> In order to fix this issue, we just need return NULL under the condition
> that both 'left' and 'curr' are NULL, meaning that no entity available.
And how would that happen? We only call pick_next_entity(.curr=NULL)
when we _know_ cfs_rq->nr_running.
> Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> kernel/sched/fair.c | 2 ++
> 1 file changed, 2 insertions(+)
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c95880e..e64c359 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3903,6 +3903,8 @@ static void clear_buddies(struct cfs_rq *cfs_rq, struct sched_entity *se)
You diff is broken, this is very much not the clear_buddies() function.
> struct sched_entity *left = __pick_first_entity(cfs_rq);
> struct sched_entity *se;
> + if (!left && !curr)
> + return NULL;
> * If curr is set we have to see if its left of the leftmost entity
> * still in the tree, provided there was anything in the tree at all.