svn commit: r321686 - in head/sys: arm/arm compat/linuxkpi/common/src dev/ow sys

[Ian Lepore]

Author: ian
Date: Sat Jul 29 17:00:23 2017
New Revision: 321686
URL: https://svnweb.freebsd.org/changeset/base/321686

Log:
  Add inline functions to convert between sbintime_t and decimal time units.
  Use them in some existing code that is vulnerable to roundoff errors.
  
  The existing constant SBT_1NS is a honeypot, luring unsuspecting folks into
  writing code such as long_timeout_ns*SBT_1NS to generate the argument for a
  sleep call.  The actual value of 1ns in sbt units is ~4.3, leading to a
  large roundoff error giving a shorter sleep than expected when multiplying
  by the trucated value of 4 in SBT_1NS.  (The evil honeypot aspect becomes
  clear after you waste a whole day figuring out why your sleeps return early.)

Modified:
  head/sys/arm/arm/mpcore_timer.c
  head/sys/compat/linuxkpi/common/src/linux_hrtimer.c
  head/sys/dev/ow/owc_gpiobus.c
  head/sys/sys/time.h

Modified: head/sys/arm/arm/mpcore_timer.c
==============================================================================
--- head/sys/arm/arm/mpcore_timer.c	Sat Jul 29 13:54:28 2017	(r321685)
+++ head/sys/arm/arm/mpcore_timer.c	Sat Jul 29 17:00:23 2017	(r321686)
@@ -351,7 +351,7 @@ attach_et(struct arm_tmr_softc *sc)
 	sc->et.et_flags = ET_FLAGS_PERIODIC | ET_FLAGS_ONESHOT | ET_FLAGS_PERCPU;
 	sc->et.et_quality = 1000;
 	sc->et.et_frequency = sc->clkfreq;
-	sc->et.et_min_period = 20 * SBT_1NS;
+	sc->et.et_min_period = nstosbt(20);
 	sc->et.et_max_period =  2 * SBT_1S;
 	sc->et.et_start = arm_tmr_start;
 	sc->et.et_stop = arm_tmr_stop;

Modified: head/sys/compat/linuxkpi/common/src/linux_hrtimer.c
==============================================================================
--- head/sys/compat/linuxkpi/common/src/linux_hrtimer.c	Sat Jul 29 13:54:28 2017	(r321685)
+++ head/sys/compat/linuxkpi/common/src/linux_hrtimer.c	Sat Jul 29 17:00:23 2017	(r321686)
@@ -101,8 +101,8 @@ linux_hrtimer_start_range_ns(struct hrtimer *hrtimer, 
 {
 
 	mtx_lock(&hrtimer->mtx);
-	callout_reset_sbt(&hrtimer->callout, time.tv64 * SBT_1NS,
-	    nsec * SBT_1NS, hrtimer_call_handler, hrtimer, 0);
+	callout_reset_sbt(&hrtimer->callout, nstosbt(time.tv64), nstosbt(nsec),
+	    hrtimer_call_handler, hrtimer, 0);
 	hrtimer->flags |= HRTIMER_ACTIVE;
 	mtx_unlock(&hrtimer->mtx);
 }

Modified: head/sys/dev/ow/owc_gpiobus.c
==============================================================================
--- head/sys/dev/ow/owc_gpiobus.c	Sat Jul 29 13:54:28 2017	(r321685)
+++ head/sys/dev/ow/owc_gpiobus.c	Sat Jul 29 17:00:23 2017	(r321686)
@@ -295,10 +295,10 @@ owc_gpiobus_read_data(device_t dev, struct ow_timing *
 	do {
 		now = sbinuptime();
 		GETPIN(sc, &sample);
-	} while ((now - then) / SBT_1US < t->t_rdv + 2 && sample == 0);
+	} while (sbttous(now - then) < t->t_rdv + 2 && sample == 0);
 	critical_exit();
 
-	if ((now - then) / SBT_1NS < t->t_rdv * 1000)
+	if (sbttons(now - then) < t->t_rdv * 1000)
 		*bit = 1;
 	else
 		*bit = 0;

Modified: head/sys/sys/time.h
==============================================================================
--- head/sys/sys/time.h	Sat Jul 29 13:54:28 2017	(r321685)
+++ head/sys/sys/time.h	Sat Jul 29 17:00:23 2017	(r321686)
@@ -128,7 +128,7 @@ bintime_shift(struct bintime *_bt, int _exp)
 #define	SBT_1M	(SBT_1S * 60)
 #define	SBT_1MS	(SBT_1S / 1000)
 #define	SBT_1US	(SBT_1S / 1000000)
-#define	SBT_1NS	(SBT_1S / 1000000000)
+#define	SBT_1NS	(SBT_1S / 1000000000) /* beware rounding, see nstosbt() */
 #define	SBT_MAX	0x7fffffffffffffffLL
 
 static __inline int
@@ -155,6 +155,53 @@ sbttobt(sbintime_t _sbt)
 	return (_bt);
 }
 
+/*
+ * Decimal<->sbt conversions.  Multiplying or dividing by SBT_1NS results in
+ * large roundoff errors which sbttons() and nstosbt() avoid.  Millisecond and
+ * microsecond functions are also provided for completeness.
+ */
+static __inline int64_t
+sbttons(sbintime_t _sbt)
+{
+
+	return ((1000000000 * _sbt) >> 32);
+}
+
+static __inline sbintime_t
+nstosbt(int64_t _ns)
+{
+
+	return ((_ns * (((uint64_t)1 << 63) / 500000000) >> 32));
+}
+
+static __inline int64_t
+sbttous(sbintime_t _sbt)
+{
+
+	return ((1000000 * _sbt) >> 32);
+}
+
+static __inline sbintime_t
+ustosbt(int64_t _us)
+{
+
+	return ((_us * (((uint64_t)1 << 63) / 500000) >> 32));
+}
+
+static __inline int64_t
+sbttoms(sbintime_t _sbt)
+{
+
+	return ((1000 * _sbt) >> 32);
+}
+
+static __inline sbintime_t
+mstosbt(int64_t _ms)
+{
+
+	return ((_ms * (((uint64_t)1 << 63) / 500) >> 32));
+}
+
 /*-
  * Background information:
  *
@@ -210,7 +257,7 @@ sbttots(sbintime_t _sbt)
 	struct timespec _ts;
 
 	_ts.tv_sec = _sbt >> 32;
-	_ts.tv_nsec = ((uint64_t)1000000000 * (uint32_t)_sbt) >> 32;
+	_ts.tv_nsec = sbttons((uint32_t)_sbt);
 	return (_ts);
 }
 
@@ -218,8 +265,7 @@ static __inline sbintime_t
 tstosbt(struct timespec _ts)
 {
 
-	return (((sbintime_t)_ts.tv_sec << 32) +
-	    (_ts.tv_nsec * (((uint64_t)1 << 63) / 500000000) >> 32));
+	return (((sbintime_t)_ts.tv_sec << 32) + nstosbt(_ts.tv_nsec));
 }
 
 static __inline struct timeval
@@ -228,7 +274,7 @@ sbttotv(sbintime_t _sbt)
 	struct timeval _tv;
 
 	_tv.tv_sec = _sbt >> 32;
-	_tv.tv_usec = ((uint64_t)1000000 * (uint32_t)_sbt) >> 32;
+	_tv.tv_usec = sbttous((uint32_t)_sbt);
 	return (_tv);
 }
 
@@ -236,8 +282,7 @@ static __inline sbintime_t
 tvtosbt(struct timeval _tv)
 {
 
-	return (((sbintime_t)_tv.tv_sec << 32) +
-	    (_tv.tv_usec * (((uint64_t)1 << 63) / 500000) >> 32));
+	return (((sbintime_t)_tv.tv_sec << 32) + ustosbt(_tv.tv_usec));
 }
 #endif /* __BSD_VISIBLE */