linux-kernel - [PATCH} - There appears to be a minor race condition in sched.c

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Message-ID: <49CAFA83.1000005@tensilica.com>
Date:	Wed, 25 Mar 2009 20:46:11 -0700
From:	Piet Delaney <piet.delaney@...silica.com>
To:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	linux-mm@...ck.org, Johannes Weiner <jw@...ix.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH} - There appears  to be a minor race condition in sched.c

Ingo, Peter:

There appears to be a minor race condition in sched.c where
you can get a division by zero. I suspect that it only shows
up when the kernel is compiled without optimization and the code
loads rq->nr_running from memory twice.

It's part of our SMP stabilization changes that I just posted to:

     git://git.kernel.org/pub/scm/linux/kernel/git/piet/xtensa-2.6.27-smp.git

I mentioned it to Johannes the other day and he suggested passing it on to you ASAP.

-------------------------------- Begin kernel/sched.c --------------------------------
index 9a1ddb8..607ee38 100644
@@ -1388,9 +1388,11 @@ static int task_hot(struct task_struct *p, u64 now, struct sched_domain *sd);
  static unsigned long cpu_avg_load_per_task(int cpu)
  {
         struct rq *rq = cpu_rq(cpu);
+       unsigned long nr_running = rq->nr_running;

-       if (rq->nr_running)
-               rq->avg_load_per_task = rq->load.weight / rq->nr_running;
+        /* Local copy of nr_running used to avoid a possible div by zero */
+       if (nr_running)
+               rq->avg_load_per_task = rq->load.weight / nr_running;

         return rq->avg_load_per_task;
  }
-------------------------------- End kernel/sched.c --------------------------------

-piet

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