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From: markspace <-@.>
Newsgroups: comp.lang.java.programmer
Subject: Re: Volatile happens before question
Date: Thu, 19 Jan 2012 13:31:36 -0800
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On 1/19/2012 12:13 PM, raphfrk@gmail.com wrote:
> On Jan 19, 6:39 pm, markspace<-@.>  wrote:
>> On 1/19/2012 10:10 AM, raph...@gmail.com wrote:
>>
>>> However, you can't say that RB happens-before WA.  Therefore, there is
>>> no happens before path between W-write and R-read.  They are
>>> effectively concurrent.
>>
>> There doesn't need to be a happens-before here, however.  It's a *read*,
>> and doesn't create a data race.  The only thing you care about is that
>> writes happen-before reads.
>
> The point is that the read has to happen before the write, since the
> read is checking if a write lock happened.
>
> In reality, something like this is allowed
>
> writeCounter.increment()
> ->  update stored in memory
> map.put()
> ->  written immediately


I'm pretty sure that's the exact opposite of what is allowed.  Remember 
that AtomicInteger::increment() is documented to have the same semantics 
as both reading and writing a volatile.  I.e., from the perspective of a 
given execution order, the write from increment() is guaranteed to 
happen first.  Writes and reads may not be reordered around it.  I don't 
think write are allowed to be interleaved with other writes this way.

>
> Seeing that writeCounter is still the old value doesn't mean that
> map.put hasn't been committed to memory yet.


It's seeing writes from the future I'm most worried about right now. 
The read in the reader thread could see (perhaps) a write from the 
future, the map.put().  I'm looking around for something like "a read 
that happens before a a volatile read may not be reordered beyond a a 
write to that same volatile."