[Xen-changelog] [linux-2.6.18-xen] xen/x86: fix for special behavior of first sys_settimeofday(NULL, &tz) invocation

["Xen patchbot-linux-2.6.18-xen" <patchbot-linux-2.6.18-xen@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1276866717 -3600
# Node ID 7b350604ce957e12146afc432c37c81c0b75e8fc
# Parent  a24d8631a0007dcb31b128452c6162ea796b05aa
xen/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@xxxxxxxxxx>
---
 arch/i386/kernel/time-xen.c |   55 ++++++++++++++++++++++++++++----------------
 kernel/time.c               |    3 ++
 2 files changed, 38 insertions(+), 20 deletions(-)

diff -r a24d8631a000 -r 7b350604ce95 arch/i386/kernel/time-xen.c
--- a/arch/i386/kernel/time-xen.c       Fri Jun 11 09:37:25 2010 +0100
+++ b/arch/i386/kernel/time-xen.c       Fri Jun 18 14:11:57 2010 +0100
@@ -114,9 +114,6 @@ static struct timespec shadow_tv;
 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);
@@ -377,6 +374,12 @@ 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.
@@ -389,7 +392,7 @@ 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);
@@ -412,6 +415,8 @@ void do_gettimeofday(struct timeval *tv)
                nsec = shadow->system_timestamp - processed_system_time;
                __normalize_time(&sec, &nsec);
                usec += (long)nsec / NSEC_PER_USEC;
+
+               monotonic_version = monotonic.version;
 
                if (unlikely(!time_values_up_to_date(cpu))) {
                        /*
@@ -434,23 +439,32 @@ 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_unlock_irqrestore(&monotonic_lock, flags);
+       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);
 
        tv->tv_sec = sec;
        tv->tv_usec = usec;
 }
 
 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)
 {
@@ -459,6 +473,11 @@ int do_settimeofday(struct timespec *tv)
        unsigned int cpu;
        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;
@@ -499,11 +518,7 @@ 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);
 
diff -r a24d8631a000 -r 7b350604ce95 kernel/time.c
--- a/kernel/time.c     Fri Jun 11 09:37:25 2010 +0100
+++ b/kernel/time.c     Fri Jun 18 14:11:57 2010 +0100
@@ -135,6 +135,9 @@ 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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