[3.8.y.z extended stable] Patch "s390/vtime: correct idle time calculation" has been added to staging queue

[Kamal Mostafa]

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

    s390/vtime: correct idle time calculation

to the linux-3.8.y-queue branch of the 3.8.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-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.14.

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

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 07821291dcb61bbf71e11b777b67d1712072d157 Mon Sep 17 00:00:00 2001
From: Martin Schwidefsky <schwidefsky at de.ibm.com>
Date: Mon, 28 Oct 2013 12:15:32 +0100
Subject: s390/vtime: correct idle time calculation

commit 4560e7c3317c7a2b370e36dadd3a3bac2ed70818 upstream.

Use the ACCESS_ONCE macro for both accesses to idle->sequence in the
loops to calculate the idle time. If only one access uses the macro,
the compiler is free to cache the value for the second access which
can cause endless loops.

Signed-off-by: Martin Schwidefsky <schwidefsky at de.ibm.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/s390/kernel/smp.c   | 4 ++--
 arch/s390/kernel/vtime.c | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c
index 7433a2f..8341c28 100644
--- a/arch/s390/kernel/smp.c
+++ b/arch/s390/kernel/smp.c
@@ -934,7 +934,7 @@ static ssize_t show_idle_count(struct device *dev,
 		idle_count = ACCESS_ONCE(idle->idle_count);
 		if (ACCESS_ONCE(idle->clock_idle_enter))
 			idle_count++;
-	} while ((sequence & 1) || (idle->sequence != sequence));
+	} while ((sequence & 1) || (ACCESS_ONCE(idle->sequence) != sequence));
 	return sprintf(buf, "%llu\n", idle_count);
 }
 static DEVICE_ATTR(idle_count, 0444, show_idle_count, NULL);
@@ -952,7 +952,7 @@ static ssize_t show_idle_time(struct device *dev,
 		idle_time = ACCESS_ONCE(idle->idle_time);
 		idle_enter = ACCESS_ONCE(idle->clock_idle_enter);
 		idle_exit = ACCESS_ONCE(idle->clock_idle_exit);
-	} while ((sequence & 1) || (idle->sequence != sequence));
+	} while ((sequence & 1) || (ACCESS_ONCE(idle->sequence) != sequence));
 	idle_time += idle_enter ? ((idle_exit ? : now) - idle_enter) : 0;
 	return sprintf(buf, "%llu\n", idle_time >> 12);
 }
diff --git a/arch/s390/kernel/vtime.c b/arch/s390/kernel/vtime.c
index e84b8b6..2fc9deb 100644
--- a/arch/s390/kernel/vtime.c
+++ b/arch/s390/kernel/vtime.c
@@ -195,7 +195,7 @@ cputime64_t s390_get_idle_time(int cpu)
 		sequence = ACCESS_ONCE(idle->sequence);
 		idle_enter = ACCESS_ONCE(idle->clock_idle_enter);
 		idle_exit = ACCESS_ONCE(idle->clock_idle_exit);
-	} while ((sequence & 1) || (idle->sequence != sequence));
+	} while ((sequence & 1) || (ACCESS_ONCE(idle->sequence) != sequence));
 	return idle_enter ? ((idle_exit ?: now) - idle_enter) : 0;
 }

--
1.8.3.2