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

Andrew Haley aph at redhat.com
Tue Jul 25 16:57:28 UTC 2017

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.

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