[PATCH 3.2 50/94] sched: Fix possible divide by zero in avg_atom() calculation

[Ben Hutchings]

3.2.62-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Mateusz Guzik <mguzik@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1402750809-31991-1-git-send-email-mguzik@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 kernel/sched_debug.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/sched_debug.c
+++ b/kernel/sched_debug.c
@@ -467,7 +467,7 @@ void proc_sched_show_task(struct task_st
 
 		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;
 

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