Re: Divide-by-zero in post_init_entity_util_avg

[Yuyang Du <yuyang.du@intel.com>]

On Thu, Jun 09, 2016 at 10:01:42AM +0100, Chris Wilson wrote:
> I've presumed commit 2b8c41daba327 ("sched/fair: Initiate a new task's
> util avg to a bounded value") to be at fault, hence the CCs. Though it
> may just be a victim.
> 
> gdb says 0x43/0x80 is
> 
>    725			if (cfs_rq->avg.util_avg != 0) {
>    726				sa->util_avg  = cfs_rq->avg.util_avg * se->load.weight;
> -> 727				sa->util_avg /= (cfs_rq->avg.load_avg + 1);
>    728	
>    729				if (sa->util_avg > cap)
>    730					sa->util_avg = cap;
>    731			} else {
> 
> I've run the same fork-heavy workload that seemed to hit the initial
> fault under kasan. kasan has not reported any errors, nor has the bug
> reoccurred after a day (earlier I had a couple of panics within a few
> hours). 
> 
> Is it possible for a race window where cfg_rq->avg.load_avg is indeed
> -1? Any evidence of other memcorruption in the above?

-1 should not be possible, sounds like a soft error.

But, a race is anyway hazardous. Thanks a lot, Chris.

--
Subject: [PATCH] sched/fair: Avoid hazardous reading cfs_rq->avg.load_avg
 without rq lock

The commit 2b8c41daba327 ("sched/fair: Initiate a new task's util avg
to a bounded value") references cfs_rq->avg.load_avg and then the value
is used as a divisor (actually cfs_rq->avg.load_avg + 1).

This race condition may cause a divide-by-zero exception. Fix it by
moving it into rq locked section.

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Yuyang Du <yuyang.du@intel.com>
---
 kernel/sched/core.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 385c947..b9f44df 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2535,10 +2535,9 @@ void wake_up_new_task(struct task_struct *p)
 	 */
 	set_task_cpu(p, select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0));
 #endif
+	rq = __task_rq_lock(p, &rf);
 	/* Post initialize new task's util average when its cfs_rq is set */
 	post_init_entity_util_avg(&p->se);
-
-	rq = __task_rq_lock(p, &rf);
 	activate_task(rq, p, 0);
 	p->on_rq = TASK_ON_RQ_QUEUED;
 	trace_sched_wakeup_new(p);
--