linux-kernel - Divide-by-zero in the 2.6.23 scheduler code

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Date:	Tue, 13 Nov 2007 20:14:55 -0500
From:	Chuck Ebbert <cebbert@...hat.com>
To:	linux-kernel <linux-kernel@...r.kernel.org>
CC:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Divide-by-zero in the 2.6.23 scheduler code

https://bugzilla.redhat.com/show_bug.cgi?id=340161

The problem code has been removed in 2.6.24. The below patch disables
SCHED_FEAT_PRECISE_CPU_LOAD which causes the offending code to be skipped
but does not prevent the user from enabling it.

The divide-by-zero is here in kernel/sched.c:

static void update_cpu_load(struct rq *this_rq)
{
	u64 fair_delta64, exec_delta64, idle_delta64, sample_interval64, tmp64;
	unsigned long total_load = this_rq->ls.load.weight;
	unsigned long this_load =  total_load;
	struct load_stat *ls = &this_rq->ls;
	int i, scale;

	this_rq->nr_load_updates++;
	if (unlikely(!(sysctl_sched_features & SCHED_FEAT_PRECISE_CPU_LOAD)))
		goto do_avg;

	/* Update delta_fair/delta_exec fields first */
	update_curr_load(this_rq);

	fair_delta64 = ls->delta_fair + 1;
	ls->delta_fair = 0;

	exec_delta64 = ls->delta_exec + 1;
	ls->delta_exec = 0;

	sample_interval64 = this_rq->clock - ls->load_update_last;
	ls->load_update_last = this_rq->clock;

	if ((s64)sample_interval64 < (s64)TICK_NSEC)
		sample_interval64 = TICK_NSEC;

	if (exec_delta64 > sample_interval64)
		exec_delta64 = sample_interval64;

	idle_delta64 = sample_interval64 - exec_delta64;

======>	tmp64 = div64_64(SCHED_LOAD_SCALE * exec_delta64, fair_delta64);
	tmp64 = div64_64(tmp64 * exec_delta64, sample_interval64);

	this_load = (unsigned long)tmp64;

do_avg:

	/* Update our load: */
	for (i = 0, scale = 1; i < CPU_LOAD_IDX_MAX; i++, scale += scale) {
		unsigned long old_load, new_load;

		/* scale is effectively 1 << i now, and >> i divides by scale */

		old_load = this_rq->cpu_load[i];
		new_load = this_load;

		this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) >> i;
	}
}


---
 kernel/sched_fair.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- linux-2.6.23.noarch.orig/kernel/sched_fair.c
+++ linux-2.6.23.noarch/kernel/sched_fair.c
@@ -93,7 +93,7 @@ unsigned int sysctl_sched_features __rea
 		SCHED_FEAT_FAIR_SLEEPERS	*1 |
 		SCHED_FEAT_SLEEPER_AVG		*0 |
 		SCHED_FEAT_SLEEPER_LOAD_AVG	*1 |
-		SCHED_FEAT_PRECISE_CPU_LOAD	*1 |
+		SCHED_FEAT_PRECISE_CPU_LOAD	*0 |
 		SCHED_FEAT_START_DEBIT		*1 |
 		SCHED_FEAT_SKIP_INITIAL		*0;
 
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