Re: PSA: If you run -current, beware!

On Wednesday, February 04, 2015 04:29:41 PM Konstantin Belousov wrote:
> On Tue, Feb 03, 2015 at 01:33:15PM -0800, Peter Wemm wrote:
> > Sometime in the Dec 10th through Jan 7th timeframe a timing bug has been
> > introduced to 11.x/head/-current.    With HZ=1000 (the default for bare
> > metal, not for a vm); the clocks stop just after 24 days of uptime.  This
> > means things like cron, sleep, timeouts etc stop working.  TCP/IP won't
> > time out or retransmit, etc etc.  It can get ugly.
> > 
> > The problem is NOT in 10.x/-stable.
> > 
> > We hit this in the freebsd.org cluster, the builds that we used are:
> > FreeBSD 11.0-CURRENT #0 r275684: Wed Dec 10 20:38:43 UTC 2014 - fine
> > FreeBSD 11.0-CURRENT #0 r276779: Wed Jan  7 18:47:09 UTC 2015 - broken
> > 
> > If you are running -current in a situation where it'll accumulate uptime,
> > you may want to take precautions.  A reboot prior to 24 days uptime (as
> > horrible a workaround as that is) will avoid it.
> > 
> > Yes, this is being worked on.
> 
> So the issue is reproducable in 3 minutes after boot with the following
> change in kern_clock.c:
> volatile int	ticks = INT_MAX - (/*hz*/1000 * 3 * 60);
> 
> It is fixed (in the proper meaning of the word, not like worked around,
> covered by paper) by the patch at the end of the mail.
> 
> We already have a story trying to enable much less ambitious option
> -fno-strict-overflow, see r259045 and the revert in r259422.  I do not
> see other way than try one more time.  Too many places in kernel
> depend on the correctly wrapping 2-complement arithmetic, among others
> are callweel and scheduler.

Ugh.

I believe I have a smoking gun that suggests that the clock-stop problem is 
caused by the clang-3.5 import on Dec 31st.

Backstory:
http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html
http://www.airs.com/blog/archives/120

I suspect that what has happened is that clang's optimizer got better at 
seeing the direct or indirect effects of integer overflow and clang (and gcc) 
take advantage of that.

I have used a slightly different change for about 10 years:

--- kern/kern_clock.c	2014-12-01 15:42:21.707911656 -0800
+++ kern/kern_clock.c	2014-12-01 15:42:21.707911656 -0800
_at__at_ -410,6 +415,11 _at__at_
 #ifdef SW_WATCHDOG
 	EVENTHANDLER_REGISTER(watchdog_list, watchdog_config, NULL, 0);
 #endif
+	/*
+	 * Arrange for ticks to go negative just 5 minutes after boot
+	 * to help catch sign problems sooner.
+	 */
+	ticks = INT_MAX - (hz * 5 * 60);
 }
 
 /*

This came about from when we had problems with integer overflow arithmetic in 
the tcp stack.

In any case, I'm in the process of adding -fwrapv and the early wraparound to 
the freebsd.org cluster builds to give it some wider exercise.

-- 
Peter Wemm - peter_at_wemm.org; peter_at_FreeBSD.org; peter_at_yahoo-inc.com; KI6FJV
UTF-8: for when a ' or ... just won\342\200\231t do\342\200\246