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

[Erik Österlund]

> On 25 Jul 2017, at 18:57, Andrew Haley <aph at redhat.com> wrote:
> 
> On 25/07/17 14:41, Erik Österlund wrote:
>> 
>> 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.
> 
> OK, I see what you're getting at.  Compilers have to be pretty
> smart to make consume work properly.

Precisely.

/Erik

> -- 
> Andrew Haley
> Java Platform Lead Engineer
> Red Hat UK Ltd. <https://www.redhat.com>
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