Re: JavaMemoryModel: Reconsidering spurious wakeups

From: Joshua Bloch (
Date: Sun Nov 23 2003 - 09:51:23 EST


   Amusingly, we just added language to the Object spec permitting
spurious wakeups
Of course we could back this out at any time before Tiger ships, but I
just thought you might like to know.


Doug Lea wrote:

>For those who haven't been following notify/interrupt/etc exchanges
>lately, the main result is that we no longer know of any intrinsic,
>compelling implementation-based reasons for allowing spurious
>wakeups. This holds for the kinds of implementations that actually
>exist and/or that seem plausible, including those adopting the various
>allowed policies surrounding early/late InterruptedException throws.
>in some cases, achieving this leverages other non-determinisms in
>specs, but not in interesting ways. There might still be some
>performance issues in avoiding some spurious interrupts in some
>implementations, but they all seem small enough to ignore.
>In which case, it becomes a pure policy issue about whether to:
>1. Ban spurious wakeups
>2. Allow spurious wakeups even though {most,all} JVMs never generate them.
>The main case for banning them is that the spec becomes simpler --
>unlike original spec, the revised version would explicitly disallow
>them. A secondary reason is that people would be able to write code
>under the reasoning "if I was notified then I must be in state X".
>(Such reasoning almost never works out in practice though.)
>The main case for allowing them is that nearly all programs using
>waits that don't recheck conditions are broken, and this would make
>them "officially" broken. Even so, production JVMs would choose to
>use techniques like those we've discussed so as not to generate
>spurious wakeups (except perhaps in a debugging/test mode). There are
>broken programs out there that people rely on, that just so happen
>never to encounter wakeups that they can't deal with, but that would
>stop working if JVMs regularly performed spurious wakeups.
>I know members of JMM list and/or JSR133 will disagree about which way
>to go here. Does anyone know how to settle this?
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