RFR (S) 8182397: Race in field updates when creating ArrayKlasses can lead to crash

[Erik Österlund]

On 2017-07-25 14:42, Andrew Haley wrote:
> On 25/07/17 12:13, Erik Österlund wrote:
>> For example, take this example pseudo code for performing what I refer
>> to as a stable load between two fields modified concurrently with
>> potential ABA issues:
>>
>> loop {
>>     x_start = load_relaxed(field_A)
>>     y = load_consume(field_B)
>>     x = load_consume(field_A)
>>     if (x_start == x) break;
>> }
>>
>> // use x->foo
> I don't understand this pseudocode.  What is the base address for field_A
> and field_B ?
>

field_A and field_B could be two different registers pointing at 
different addresses - i.e. they are arbitrary pointers. The key in this 
example is that field_A is reloaded, and then we compare if the reloaded 
value is equal to the original value (with a possible ABA problem), and 
stop the loop then. But the original and reloaded value could reside in 
different registers, and when we continue using x->foo afterwards, the 
compiler could elect to use either one of the two registers as base 
pointers in the dereference - either the one from the reloaded value of 
field_A or for the original value, as they are equal to each other. 
Normally that is totally fine, but had the compiler known that the 
reload had consume semantics, it would have known that it definitely has 
to use that register that the reload went into when dereferencing x->foo 
after the loop, despite the fact that it is arithmetically equal to the 
other register.

Hope that makes more sense.

Thanks,
/Erik