[3.11.y.z extended stable] Patch "sched: Fix possible divide by zero in avg_atom() calculation" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    sched: Fix possible divide by zero in avg_atom() calculation

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From ece7db3abd9bdfe1edcb3619d35fb545aeaf8d5a Mon Sep 17 00:00:00 2001
From: Mateusz Guzik <mguzik at redhat.com>
Date: Sat, 14 Jun 2014 15:00:09 +0200
Subject: sched: Fix possible divide by zero in avg_atom() calculation

commit b0ab99e7736af88b8ac1b7ae50ea287fffa2badc upstream.

proc_sched_show_task() does:

  if (nr_switches)
	do_div(avg_atom, nr_switches);

nr_switches is unsigned long and do_div truncates it to 32 bits, which
means it can test non-zero on e.g. x86-64 and be truncated to zero for
division.

Fix the problem by using div64_ul() instead.

As a side effect calculations of avg_atom for big nr_switches are now correct.

Signed-off-by: Mateusz Guzik <mguzik at redhat.com>
Signed-off-by: Peter Zijlstra <peterz at infradead.org>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Link: http://lkml.kernel.org/r/1402750809-31991-1-git-send-email-mguzik@redhat.com
Signed-off-by: Ingo Molnar <mingo at kernel.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 kernel/sched/debug.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 631daf2612c1..8304ee67c56b 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -554,7 +554,7 @@ void proc_sched_show_task(struct task_struct *p, struct seq_file *m)

 		avg_atom = p->se.sum_exec_runtime;
 		if (nr_switches)
-			do_div(avg_atom, nr_switches);
+			avg_atom = div64_ul(avg_atom, nr_switches);
 		else
 			avg_atom = -1LL;

--
1.9.1