[infinispan-dev] JGRP-1877

[Alan Field]

Hey Bela,

> Just a quick heads up. I'm currently working on
> https://issues.jboss.org/browse/JGRP-1877, which it critical as it may:
> - cause RPCs to return prematurely (possibly with a TimeoutException), or
> - cause RPCs to blocks for a long time (pick which one is worse :-))

How frequently can these errors occur? Is this something that is not very likely to happen or something that requires an external action to trigger it? (i.e. changing the time via NTP) Just trying to determine the priority of this issue.

Thanks,
Alan

----- Original Message -----
> From: "Bela Ban" <bban at redhat.com>
> To: infinispan-dev at lists.jboss.org
> Sent: Wednesday, September 10, 2014 12:05:11 PM
> Subject: [infinispan-dev] JGRP-1877
> 
> Just a quick heads up. I'm currently working on
> https://issues.jboss.org/browse/JGRP-1877, which it critical as it may:
> - cause RPCs to return prematurely (possibly with a TimeoutException), or
> - cause RPCs to blocks for a long time (pick which one is worse :-))
> 
> This is due to my misunderstanding of the semantics of
> System.nanoTime(), I frequently have code like this, which computes a
> future deadline for a timeout:
> 
>              long wait_time=TimeUnit.NANOSECONDS.convert(timeout,
> TimeUnit.MILLISECONDS);
>              final long target_time=System.nanoTime() + wait_time;
>              while(wait_time > 0 && !hasResult) { /* Wait for responses: */
>                  wait_time=target_time - System.nanoTime();
>                  if(wait_time > 0) {
>                      try {cond.await(wait_time, TimeUnit.NANOSECONDS);}
> catch(Exception e) {}
>                  }
>              }
>              if(!hasResult && wait_time <= 0)
>                  throw new TimeoutException();
> 
> Variable target_time can possibly become *negative* if nanoTime()
> returns a negative value. If so, hasResult is false and wait_time
> negative, and therefore a TimeoutException would be thrown !
> 
> While I'm at it, I'll also fix my uses of System.currentTimeMillis(),
> and replace it with nanoTime(). Our good friend Erik has run into issues
> with RPCs (using currentTimeMillis()) hanging forever when their
> NTP-based servers adjusted the time .... backwards !
> 
> Please be aware of nanoTime() in your own code, e.g.
> long t0=nanoTime();
> ...
> long t1=nanoTime();
> 
> It is *not* guaranteed that t1 > t0 because of numeric overflow (t0
> might be Long.MAX_VALUE-1 and t1 Long.MAX_VALUE +2 !). The only way to
> compare them is t1 - t0 > 0 (t1 is more recent) or < 0 t0 is more recent.
> 
> Just thought I wanted to pass this on, in case somebody made the same
> stupid mistake...
> 
> Thanks to David Lloyd for pointing this out !
> 
> --
> Bela Ban, JGroups lead (http://www.jgroups.org)
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