[3.5.y.z,extended,stable] Patch "s390/timer: avoid overflow when programming clock comparator" has been added to staging queue

Message ID 1360947774-25107-1-git-send-email-herton.krzesinski@canonical.com
State New
Headers show

Commit Message

Herton Ronaldo Krzesinski Feb. 15, 2013, 5:02 p.m.
This is a note to let you know that I have just added a patch titled

    s390/timer: avoid overflow when programming clock comparator

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:


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

For more information about the 3.5.y.z tree, see



From 691d92ad137319e5dd84318fd1ed554bb265f30a Mon Sep 17 00:00:00 2001
From: Heiko Carstens <heiko.carstens@de.ibm.com>
Date: Tue, 29 Jan 2013 09:16:28 +0100
Subject: [PATCH] s390/timer: avoid overflow when programming clock comparator

commit d911e03d097bdc01363df5d81c43f69432eb785c upstream.

Since ed4f209 "s390/time: fix sched_clock() overflow" a new helper function
is used to avoid overflows when converting TOD format values to nanosecond
The kvm interrupt code formerly however only worked by accident because of
an overflow. It tried to program a timer that would expire in more than ~29
years. Because of the old TOD-to-nanoseconds overflow bug the real expiry
value however was much smaller, but now it isn't anymore.
This however triggers yet another bug in the function that programs the clock
comparator s390_next_ktime(): if the absolute "expires" value is after 2042
this will result in an overflow and the programmed value is lower than the
current TOD value which immediatly triggers a clock comparator (= timer)
Since the timer isn't expired it will be programmed immediately again and so
on... the result is a dead system.
To fix this simply program the maximum possible value if an overflow is

Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com>
 arch/s390/kernel/time.c |    3 +++
 1 file changed, 3 insertions(+)



diff --git a/arch/s390/kernel/time.c b/arch/s390/kernel/time.c
index c5531db..747ab28 100644
--- a/arch/s390/kernel/time.c
+++ b/arch/s390/kernel/time.c
@@ -121,6 +121,9 @@  static int s390_next_ktime(ktime_t expires,
 	nsecs = ktime_to_ns(ktime_add(timespec_to_ktime(ts), expires));
 	do_div(nsecs, 125);
 	S390_lowcore.clock_comparator = sched_clock_base_cc + (nsecs << 9);
+	/* Program the maximum value if we have an overflow (== year 2042) */
+	if (unlikely(S390_lowcore.clock_comparator < sched_clock_base_cc))
+		S390_lowcore.clock_comparator = -1ULL;
 	return 0;