s390/idle: fix cpu idle time calculation

The idle time reported in /proc/stat sometimes incorrectly contains
huge values on s390. This is caused by a bug in arch_cpu_idle_time().

The kernel tries to figure out when a different cpu entered idle by
accessing its per-cpu data structure. There is an ordering problem: if
the remote cpu has an idle_enter value which is not zero, and an
idle_exit value which is zero, it is assumed it is idle since
"now". The "now" timestamp however is taken before the idle_enter
value is read.

Which in turn means that "now" can be smaller than idle_enter of the
remote cpu. Unconditionally subtracting idle_enter from "now" can thus
lead to a negative value (aka large unsigned value).

Fix this by moving the get_tod_clock() invocation out of the
loop. While at it also make the code a bit more readable.

A similar bug also exists for show_idle_time(). Fix this is as well.

Cc: <stable@vger.kernel.org>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

diff --git a/arch/s390/kernel/idle.c b/arch/s390/kernel/idle.c
index b9d8fe45737aa2e529cb9eec244349e121803858..8f8456816d83e890a3f3150ccec184f21810a509 100644 (file)
--- a/arch/s390/kernel/idle.c
+++ b/arch/s390/kernel/idle.c
@@ -69,18 +69,26 @@ DEVICE_ATTR(idle_count, 0444, show_idle_count, NULL);
 static ssize_t show_idle_time(struct device *dev,
                                struct device_attribute *attr, char *buf)
 {
+       unsigned long long now, idle_time, idle_enter, idle_exit, in_idle;
        struct s390_idle_data *idle = &per_cpu(s390_idle, dev->id);
-       unsigned long long now, idle_time, idle_enter, idle_exit;
        unsigned int seq;
 
        do {
-               now = get_tod_clock();
                seq = read_seqcount_begin(&idle->seqcount);
                idle_time = READ_ONCE(idle->idle_time);
                idle_enter = READ_ONCE(idle->clock_idle_enter);
                idle_exit = READ_ONCE(idle->clock_idle_exit);
        } while (read_seqcount_retry(&idle->seqcount, seq));
-       idle_time += idle_enter ? ((idle_exit ? : now) - idle_enter) : 0;
+       in_idle = 0;
+       now = get_tod_clock();
+       if (idle_enter) {
+               if (idle_exit) {
+                       in_idle = idle_exit - idle_enter;
+               } else if (now > idle_enter) {
+                       in_idle = now - idle_enter;
+               }
+       }
+       idle_time += in_idle;
        return sprintf(buf, "%llu\n", idle_time >> 12);
 }
 DEVICE_ATTR(idle_time_us, 0444, show_idle_time, NULL);
@@ -88,17 +96,24 @@ DEVICE_ATTR(idle_time_us, 0444, show_idle_time, NULL);
 u64 arch_cpu_idle_time(int cpu)
 {
        struct s390_idle_data *idle = &per_cpu(s390_idle, cpu);
-       unsigned long long now, idle_enter, idle_exit;
+       unsigned long long now, idle_enter, idle_exit, in_idle;
        unsigned int seq;
 
        do {
-               now = get_tod_clock();
                seq = read_seqcount_begin(&idle->seqcount);
                idle_enter = READ_ONCE(idle->clock_idle_enter);
                idle_exit = READ_ONCE(idle->clock_idle_exit);
        } while (read_seqcount_retry(&idle->seqcount, seq));
-
-       return cputime_to_nsecs(idle_enter ? ((idle_exit ?: now) - idle_enter) : 0);
+       in_idle = 0;
+       now = get_tod_clock();
+       if (idle_enter) {
+               if (idle_exit) {
+                       in_idle = idle_exit - idle_enter;
+               } else if (now > idle_enter) {
+                       in_idle = now - idle_enter;
+               }
+       }
+       return cputime_to_nsecs(in_idle);
 }
 
 void arch_cpu_idle_enter(void)