x86: tsc prevent time going backwards

[sfrench/cifs-2.6.git] / arch / x86 / kernel / tsc_32.c

diff --git a/arch/x86/kernel/tsc_32.c b/arch/x86/kernel/tsc_32.c
index f14cfd9d1f94c1a634833da25e86eea9cb43b864..d7498b34c8e9aa8069bbbfd95cdb65a7965fe882 100644 (file)
--- a/arch/x86/kernel/tsc_32.c
+++ b/arch/x86/kernel/tsc_32.c
@@ -287,14 +287,27 @@ core_initcall(cpufreq_tsc);
 /* clock source code */
 
 static unsigned long current_tsc_khz = 0;
+static struct clocksource clocksource_tsc;
 
+/*
+ * We compare the TSC to the cycle_last value in the clocksource
+ * structure to avoid a nasty time-warp issue. This can be observed in
+ * a very small window right after one CPU updated cycle_last under
+ * xtime lock and the other CPU reads a TSC value which is smaller
+ * than the cycle_last reference value due to a TSC which is slighty
+ * behind. This delta is nowhere else observable, but in that case it
+ * results in a forward time jump in the range of hours due to the
+ * unsigned delta calculation of the time keeping core code, which is
+ * necessary to support wrapping clocksources like pm timer.
+ */
 static cycle_t read_tsc(void)
 {
        cycle_t ret;
 
        rdtscll(ret);
 
-       return ret;
+       return ret >= clocksource_tsc.cycle_last ?
+               ret : clocksource_tsc.cycle_last;
 }
 
 static struct clocksource clocksource_tsc = {