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List:       git-commits-head
Subject:    sched: prevent divide by zero error in cpu_avg_load_per_task
From:       Linux Kernel Mailing List <linux-kernel () vger ! kernel ! org>
Date:       2008-11-30 22:00:41
Message-ID: 200811302200.mAUM0fbS004186 () hera ! kernel ! org
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Gitweb:     http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=4cd4262034849da01eb88659af677b69f8169f06
Commit:     4cd4262034849da01eb88659af677b69f8169f06
Parent:     ee2f6cc7f9ea2542ad46070ed62ba7aa04d08871
Author:     Steven Rostedt <rostedt@goodmis.org>
AuthorDate: Wed Nov 26 21:04:24 2008 -0500
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Thu Nov 27 10:29:52 2008 +0100

    sched: prevent divide by zero error in cpu_avg_load_per_task
    
    Impact: fix divide by zero crash in scheduler rebalance irq
    
    While testing the branch profiler, I hit this crash:
    
    divide error: 0000 [#1] PREEMPT SMP
    [...]
    RIP: 0010:[<ffffffff8024a008>]  [<ffffffff8024a008>] cpu_avg_load_per_task+0x50/0x7f
    [...]
    Call Trace:
     <IRQ> <0> [<ffffffff8024fd43>] find_busiest_group+0x3e5/0xcaa
     [<ffffffff8025da75>] rebalance_domains+0x2da/0xa21
     [<ffffffff80478769>] ? find_next_bit+0x1b2/0x1e6
     [<ffffffff8025e2ce>] run_rebalance_domains+0x112/0x19f
     [<ffffffff8026d7c2>] __do_softirq+0xa8/0x232
     [<ffffffff8020ea7c>] call_softirq+0x1c/0x3e
     [<ffffffff8021047a>] do_softirq+0x94/0x1cd
     [<ffffffff8026d5eb>] irq_exit+0x6b/0x10e
     [<ffffffff8022e6ec>] smp_apic_timer_interrupt+0xd3/0xff
     [<ffffffff8020e4b3>] apic_timer_interrupt+0x13/0x20
    
    The code for cpu_avg_load_per_task has:
    
    	if (rq->nr_running)
    		rq->avg_load_per_task = rq->load.weight / rq->nr_running;
    
    The runqueue lock is not held here, and there is nothing that prevents
    the rq->nr_running from going to zero after it passes the if condition.
    
    The branch profiler simply made the race window bigger.
    
    This patch saves off the rq->nr_running to a local variable and uses that
    for both the condition and the division.
    
    Signed-off-by: Steven Rostedt <srostedt@redhat.com>
    Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/sched.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 9b1e793..700aa9a 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1453,9 +1453,10 @@ 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;
+	if (nr_running)
+		rq->avg_load_per_task = rq->load.weight / nr_running;
 	else
 		rq->avg_load_per_task = 0;
 
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