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From 4a44a54b3caf77923f0e3f1d5bdf5eda6ef07f62 Mon Sep 17 00:00:00 2001
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From: Chris MacGregor <chrismacgregor@google.com>
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Date: Thu, 27 Feb 2014 10:40:59 -0800
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Subject: [PATCH] hwclock: fix possible hang and other
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 set_hardware_clock_exact() issues
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In sys-utils/hwclock.c, set_hardware_clock_exact() has some problems when the
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process gets pre-empted (for more than 100ms) before reaching the time for
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which it waits:
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1. The "continue" statement causes execution to skip the final tdiff
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assignment at the end of the do...while loop, leading to the while condition
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using the wrong value of tdiff, and thus always exiting the loop once
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newhwtime != sethwtime (e.g., after 1 second).  This masks bug # 2, below.
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2. The previously-existing bug is that because it starts over waiting for the
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desired time whenever two successive calls to gettimeofday() return values >
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100ms apart, the loop will never terminate unless the process holds the CPU
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(without losing it for more than 100ms) for at least 500ms.  This can happen
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on a heavily loaded machine or on a virtual machine (or on a heavily loaded
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virtual machine).  This has been observed to occur, preventing a machine from
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completing the shutdown or reboot process due to a "hwclock --systohc" call in
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a shutdown script.
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The new implementation presented in this patch takes a somewhat different
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approach, intended to accomplish the same goals:
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It computes the desired target system time (at which the requested hardware
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clock time will be applied to the hardware clock), and waits for that time to
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arrive.  If it misses the time (such as due to being pre-empted for too long),
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it recalculates the target time, and increases the tolerance (how late it can
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be relative to the target time, and still be "close enough".  Thus, if all is
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well, the time will be set *very* precisely.  On a machine where the hwclock
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process is repeatedly pre-empted, it will set the time as precisely as is
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possible under the conditions present on that particular machine.  In any
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case, it will always terminate eventually (and pretty quickly); it will never
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hang forever.
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[kzak@redhat.com: - tiny coding style changes]
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Signed-off-by: Chris MacGregor <chrismacgregor@google.com>
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Signed-off-by: Karel Zak <kzak@redhat.com>
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---
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 sys-utils/hwclock.c | 170 ++++++++++++++++++++++++++++++++++++++++------------
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 1 file changed, 131 insertions(+), 39 deletions(-)
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diff --git a/sys-utils/hwclock.c b/sys-utils/hwclock.c
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index 30660d4..395b5c3 100644
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--- a/sys-utils/hwclock.c
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+++ b/sys-utils/hwclock.c
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@@ -125,7 +125,7 @@ struct adjtime {
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  * We are running in debug mode, wherein we put a lot of information about
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  * what we're doing to standard output.
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  */
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-bool debug;
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+int debug;
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 /* Workaround for Award 4.50g BIOS bug: keep the year in a file. */
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 bool badyear;
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@@ -526,43 +526,141 @@ set_hardware_clock_exact(const time_t sethwtime,
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 			 const struct timeval refsystime,
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 			 const bool universal, const bool testing)
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 {
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-	time_t newhwtime = sethwtime;
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-	struct timeval beginsystime, nowsystime;
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-	double tdiff;
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-	int time_resync = 1;
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-
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 	/*
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-	 * Now delay some more until Hardware Clock time newhwtime arrives.
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-	 * The 0.5 s is because the Hardware Clock always sets to your set
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-	 * time plus 500 ms (because it is designed to update to the next
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-	 * second precisely 500 ms after you finish the setting).
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+	 * The Hardware Clock can only be set to any integer time plus one
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+	 * half second.	 The integer time is required because there is no
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+	 * interface to set or get a fractional second.	 The additional half
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+	 * second is because the Hardware Clock updates to the following
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+	 * second precisely 500 ms (not 1 second!) after you release the
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+	 * divider reset (after setting the new time) - see description of
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+	 * DV2, DV1, DV0 in Register A in the MC146818A data sheet (and note
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+	 * that although that document doesn't say so, real-world code seems
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+	 * to expect that the SET bit in Register B functions the same way).
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+	 * That means that, e.g., when you set the clock to 1:02:03, it
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+	 * effectively really sets it to 1:02:03.5, because it will update to
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+	 * 1:02:04 only half a second later.  Our caller passes the desired
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+	 * integer Hardware Clock time in sethwtime, and the corresponding
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+	 * system time (which may have a fractional part, and which may or may
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+	 * not be the same!) in refsystime.  In an ideal situation, we would
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+	 * then apply sethwtime to the Hardware Clock at refsystime+500ms, so
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+	 * that when the Hardware Clock ticks forward to sethwtime+1s half a
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+	 * second later at refsystime+1000ms, everything is in sync.  So we
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+	 * spin, waiting for gettimeofday() to return a time at or after that
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+	 * time (refsystime+500ms) up to a tolerance value, initially 1ms.  If
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+	 * we miss that time due to being preempted for some other process,
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+	 * then we increase the margin a little bit (initially 1ms, doubling
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+	 * each time), add 1 second (or more, if needed to get a time that is
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+	 * in the future) to both the time for which we are waiting and the
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+	 * time that we will apply to the Hardware Clock, and start waiting
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+	 * again.
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+	 * 
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+	 * For example, the caller requests that we set the Hardware Clock to
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+	 * 1:02:03, with reference time (current system time) = 6:07:08.250.
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+	 * We want the Hardware Clock to update to 1:02:04 at 6:07:09.250 on
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+	 * the system clock, and the first such update will occur 0.500
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+	 * seconds after we write to the Hardware Clock, so we spin until the
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+	 * system clock reads 6:07:08.750.  If we get there, great, but let's
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+	 * imagine the system is so heavily loaded that our process is
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+	 * preempted and by the time we get to run again, the system clock
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+	 * reads 6:07:11.990.  We now want to wait until the next xx:xx:xx.750
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+	 * time, which is 6:07:12.750 (4.5 seconds after the reference time),
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+	 * at which point we will set the Hardware Clock to 1:02:07 (4 seconds
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+	 * after the originally requested time).  If we do that successfully,
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+	 * then at 6:07:13.250 (5 seconds after the reference time), the
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+	 * Hardware Clock will update to 1:02:08 (5 seconds after the
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+	 * originally requested time), and all is well thereafter.
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 	 */
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-	do {
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-		if (time_resync) {
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-			gettimeofday(&beginsystime, NULL);
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-			tdiff = time_diff(beginsystime, refsystime);
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-			newhwtime = sethwtime + (int)(tdiff + 0.5);
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-			if (debug)
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-				printf(_
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-				       ("Time elapsed since reference time has been %.6f seconds.\n"
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-					"Delaying further to reach the new time.\n"),
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-				       tdiff);
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-			time_resync = 0;
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+
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+	time_t newhwtime = sethwtime;
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+	double target_time_tolerance_secs = 0.001;  /* initial value */
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+	double tolerance_incr_secs = 0.001;	    /* initial value */
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+	const double RTC_SET_DELAY_SECS = 0.5;	    /* 500 ms */
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+	const struct timeval RTC_SET_DELAY_TV = { 0, RTC_SET_DELAY_SECS * 1E6 };
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+
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+	struct timeval targetsystime;
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+	struct timeval nowsystime;
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+	struct timeval prevsystime = refsystime;
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+	double deltavstarget;
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+
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+	timeradd(&refsystime, &RTC_SET_DELAY_TV, &targetsystime);
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+
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+	while (1) {
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+		double ticksize;
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+
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+		/* FOR TESTING ONLY: inject random delays of up to 1000ms */
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+		if (debug >= 10) {
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+			int usec = random() % 1000000;
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+			printf(_("sleeping ~%d usec\n"), usec);
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+			usleep(usec);
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 		}
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 		gettimeofday(&nowsystime, NULL);
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-		tdiff = time_diff(nowsystime, beginsystime);
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-		if (tdiff < 0) {
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-			time_resync = 1;	/* probably backward time reset */
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-			continue;
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-		}
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-		if (tdiff > 0.1) {
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-			time_resync = 1;	/* probably forward time reset */
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-			continue;
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+		deltavstarget = time_diff(nowsystime, targetsystime);
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+		ticksize = time_diff(nowsystime, prevsystime);
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+		prevsystime = nowsystime;
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+
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+		if (ticksize < 0) {
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+			if (debug)
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+				printf(_("time jumped backward %.6f seconds "
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+					 "to %ld.%06d - retargeting\n"),
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+				       ticksize, (long)nowsystime.tv_sec,
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+				       (int)nowsystime.tv_usec);
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+			/* The retarget is handled at the end of the loop. */
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+		} else if (deltavstarget < 0) {
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+			/* deltavstarget < 0 if current time < target time */
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+			if (debug >= 2)
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+				printf(_("%ld.%06d < %ld.%06d (%.6f)\n"),
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+				       (long)nowsystime.tv_sec,
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+				       (int)nowsystime.tv_usec,
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+				       (long)targetsystime.tv_sec,
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+				       (int)targetsystime.tv_usec,
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+				       deltavstarget);
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+			continue;  /* not there yet - keep spinning */
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+		} else if (deltavstarget <= target_time_tolerance_secs) {
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+			/* Close enough to the target time; done waiting. */
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+			break;
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+		} else /* (deltavstarget > target_time_tolerance_secs) */ {
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+			/*
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+			 * We missed our window.  Increase the tolerance and
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+			 * aim for the next opportunity.
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+			 */
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+			if (debug)
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+				printf(_("missed it - %ld.%06d is too far "
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+					 "past %ld.%06d (%.6f > %.6f)\n"),
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+				       (long)nowsystime.tv_sec,
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+				       (int)nowsystime.tv_usec,
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+				       (long)targetsystime.tv_sec,
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+				       (int)targetsystime.tv_usec,
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+				       deltavstarget,
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+				       target_time_tolerance_secs);
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+			target_time_tolerance_secs += tolerance_incr_secs;
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+			tolerance_incr_secs *= 2;
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 		}
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-		beginsystime = nowsystime;
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-		tdiff = time_diff(nowsystime, refsystime);
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-	} while (newhwtime == sethwtime + (int)(tdiff + 0.5));
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+
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+		/*
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+		 * Aim for the same offset (tv_usec) within the second in
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+		 * either the current second (if that offset hasn't arrived
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+		 * yet), or the next second.
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+		 */
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+		if (nowsystime.tv_usec < targetsystime.tv_usec)
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+			targetsystime.tv_sec = nowsystime.tv_sec;
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+		else
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+			targetsystime.tv_sec = nowsystime.tv_sec + 1;
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+	}
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+
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+	newhwtime = sethwtime
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+		    + (int)(time_diff(nowsystime, refsystime)
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+			    - RTC_SET_DELAY_SECS /* don't count this */
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+			    + 0.5 /* for rounding */);
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+	if (debug)
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+		printf(_("%ld.%06d is close enough to %ld.%06d (%.6f < %.6f)\n"
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+			 "Set RTC to %ld (%ld + %d; refsystime = %ld.%06d)\n"),
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+		       (long)nowsystime.tv_sec, (int)nowsystime.tv_usec,
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+		       (long)targetsystime.tv_sec, (int)targetsystime.tv_usec,
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+		       deltavstarget, target_time_tolerance_secs,
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+		       (long)newhwtime, (long)sethwtime,
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+		       (int)(newhwtime - sethwtime),
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+		       (long)refsystime.tv_sec, (int)refsystime.tv_usec);
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 	set_hardware_clock(newhwtime, universal, testing);
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 }
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@@ -1636,7 +1734,7 @@ int main(int argc, char **argv)
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 		switch (c) {
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 		case 'D':
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-			debug = TRUE;
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+			++debug;
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 			break;
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 		case 'a':
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 			adjust = TRUE;
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@@ -1953,10 +2051,4 @@ void __attribute__((__noreturn__)) hwaudit_exit(int status)
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  *
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  * hwclock uses this method, and considers the Hardware Clock to have
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  * infinite precision.
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- *
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- * TODO: Enhancements needed:
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- *
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- *  - When waiting for whole second boundary in set_hardware_clock_exact,
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- *    fail if we miss the goal by more than .1 second, as could happen if we
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- *    get pre-empted (by the kernel dispatcher).
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  */
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-- 
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1.9.3
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