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Subject: Re: Volatile happens before question
Date: Tue, 17 Jan 2012 08:50:22 -0800
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On 1/17/2012 8:17 AM, Peter Duniho wrote:

>> int b = 0;
>> volatile boolean a = false;
>> ...
>> ...
>> a = true;
>> b = 1;

>> (set a = true) happens before (set b = 1)
>
> It's this last assertion that fails.


I believe the original assertion is true, but in code he was assuming 
the converse. (*)  That is a is true before b is 1, but just because a 
is true doesn't mean that b has been set to 1 yet.  There is a physical 
time gap in between those events. (**)

In other words, the JLS does not say that:

a is true iff b is 1.

(iff = "if and only if").

The JLS does say:

If b is 1, then a has also been set to true. (a happens before b.)

The JLS does not say:

If a is true, then b has been also set to 1.

That last statement could be false and there could still be a valid 
happens-before relationship between write(a<-true) and write(b<-1). 
There is a time gap between a being written and b being written where 
you could see a==true but b==0.

As a general rule of thumb, you always read the volatile FIRST and write 
the volatile LAST.  Here in the 2nd example, a was read first and the 
volatile (b) was read after, so that's why it didn't work.



(*) I'm wondering if the proper word is reflexive rather than converse. 
  That is, just because

(b=1) therefore (a=true) is true, does not mean

(a=true) therefore (b=1) is true (it's not).

Any help here?


(**) These are terrible variable names for discussion a problem in 
English.  "a" looks like the article a and b is just as bad.  Use 
something longer and more obvious next time.  I can scarcely follow my 
own writing, sheesh.