Jan Beulich
2010-Jun-18 10:00 UTC
[Xen-devel] [PATCH] linux/x86: fix for special behavior of first sys_settimeofday(NULL, &tz) invocation
The data Xen''s time implementation maintains to make do_gettimeofday()
return values monotonic needs to be reset not only during normal
do_gettimeofday() invocations, but also when the clock gets warped
due to the hardware (CMOS) clock running on local (rather than UTC)
time.
Additionally there was a time window in do_gettimeofday() (between
the end of the xtime read loop and the acquiring of the monotonicity
data lock) where, if on another processor do_settimeofday() would
execute to completion, the zeroes written by the latter could get
overwritten by the former with values obtained before the time was
updated. This now gets prevented by maintaining a version for the
monotonicity data.
Signed-off-by: Jan Beulich <jbeulich@novell.com>
--- sle10sp3-2010-06-17.orig/arch/i386/kernel/time-xen.c 2009-07-27
10:22:10.000000000 +0200
+++ sle10sp3-2010-06-17/arch/i386/kernel/time-xen.c 2010-06-18
08:21:37.000000000 +0200
@@ -119,9 +119,6 @@ static DEFINE_PER_CPU(struct shadow_time
static struct timespec shadow_tv;
static u32 shadow_tv_version;
-static struct timeval monotonic_tv;
-static spinlock_t monotonic_lock = SPIN_LOCK_UNLOCKED;
-
/* Keep track of last time we did processing/updating of jiffies and xtime. */
static u64 processed_system_time; /* System time (ns) at last processing. */
static DEFINE_PER_CPU(u64, processed_system_time);
@@ -384,6 +381,12 @@ void rtc_cmos_write(unsigned char val, u
}
EXPORT_SYMBOL(rtc_cmos_write);
+static struct {
+ spinlock_t lock;
+ struct timeval tv;
+ u32 version;
+} monotonic = { .lock = SPIN_LOCK_UNLOCKED };
+
/*
* This version of gettimeofday has microsecond resolution
* and better than microsecond precision on fast x86 machines with TSC.
@@ -396,7 +399,7 @@ void do_gettimeofday(struct timeval *tv)
s64 nsec;
unsigned int cpu;
struct shadow_time_info *shadow;
- u32 local_time_version;
+ u32 local_time_version, monotonic_version;
cpu = get_cpu();
shadow = &per_cpu(shadow_time, cpu);
@@ -420,6 +423,8 @@ void do_gettimeofday(struct timeval *tv)
__normalize_time(&sec, &nsec);
usec += (long)nsec / NSEC_PER_USEC;
+ monotonic_version = monotonic.version;
+
if (unlikely(!time_values_up_to_date(cpu))) {
/*
* We may have blocked for a long time,
@@ -441,17 +446,16 @@ void do_gettimeofday(struct timeval *tv)
sec++;
}
- spin_lock_irqsave(&monotonic_lock, flags);
- if ((sec > monotonic_tv.tv_sec) ||
- ((sec == monotonic_tv.tv_sec) && (usec >
monotonic_tv.tv_usec)))
- {
- monotonic_tv.tv_sec = sec;
- monotonic_tv.tv_usec = usec;
- } else {
- sec = monotonic_tv.tv_sec;
- usec = monotonic_tv.tv_usec;
+ spin_lock_irqsave(&monotonic.lock, flags);
+ if (unlikely(sec < monotonic.tv.tv_sec) ||
+ (sec == monotonic.tv.tv_sec && usec <= monotonic.tv.tv_usec)) {
+ sec = monotonic.tv.tv_sec;
+ usec = monotonic.tv.tv_usec;
+ } else if (likely(monotonic_version == monotonic.version)) {
+ monotonic.tv.tv_sec = sec;
+ monotonic.tv.tv_usec = usec;
}
- spin_unlock_irqrestore(&monotonic_lock, flags);
+ spin_unlock_irqrestore(&monotonic.lock, flags);
tv->tv_sec = sec;
tv->tv_usec = usec;
@@ -459,6 +463,16 @@ void do_gettimeofday(struct timeval *tv)
EXPORT_SYMBOL(do_gettimeofday);
+/* Reset monotonic gettimeofday() timeval. */
+static inline void monotonic_reset(void)
+{
+ spin_lock(&monotonic.lock);
+ monotonic.tv.tv_sec = 0;
+ monotonic.tv.tv_usec = 0;
+ ++monotonic.version;
+ spin_unlock(&monotonic.lock);
+}
+
int do_settimeofday(struct timespec *tv)
{
time_t sec;
@@ -467,6 +481,11 @@ int do_settimeofday(struct timespec *tv)
struct shadow_time_info *shadow;
struct xen_platform_op op;
+ if (unlikely(!tv)) {
+ monotonic_reset();
+ return 0;
+ }
+
if ((unsigned long)tv->tv_nsec >= NSEC_PER_SEC)
return -EINVAL;
@@ -506,11 +525,7 @@ int do_settimeofday(struct timespec *tv)
}
ntp_clear();
- /* Reset monotonic gettimeofday() timeval. */
- spin_lock(&monotonic_lock);
- monotonic_tv.tv_sec = 0;
- monotonic_tv.tv_usec = 0;
- spin_unlock(&monotonic_lock);
+ monotonic_reset();
write_sequnlock_irq(&xtime_lock);
--- sle10sp3-2010-06-17.orig/kernel/time.c 2010-06-18 08:19:43.000000000 +0200
+++ sle10sp3-2010-06-17/kernel/time.c 2010-06-17 17:46:07.000000000 +0200
@@ -136,6 +136,9 @@ static inline void warp_clock(void)
wall_to_monotonic.tv_sec -= sys_tz.tz_minuteswest * 60;
xtime.tv_sec += sys_tz.tz_minuteswest * 60;
time_interpolator_reset();
+#if defined(CONFIG_XEN) && defined(CONFIG_X86)
+ do_settimeofday(NULL);
+#endif
write_sequnlock_irq(&xtime_lock);
clock_was_set();
}
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