RFR: 8272807: Permit use of memory concurrent with pretouch

[Kim Barrett]

On Fri, 4 Feb 2022 12:07:32 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> src/hotspot/share/runtime/os.cpp line 1771:
>> 
>>> 1769:       Atomic::add(reinterpret_cast<int*>(cur), 0, memory_order_relaxed);
>>> 1770:       if (cur == last) break;
>>> 1771:     }
>> 
>> Took me some time to check the loop. Suggestion:
>> 
>> 
>>     do {
>>       Atomic::add(reinterpret_cast<int*>(cur), 0, memory_order_relaxed);
>>       cur += page_size;
>>     } while (cur < last);
>
> Intentionally didn't write it that way, because of the potential for UB pointer arithmetic overflow in `cur += page_size`.

Also, that won't touch the last page when first page != last page, unless the last page is made exclusive, which calculation could also suffer from UB pointer arithmetic overflow.  Of course, the caller could suffer from that too; we can't so easily fix that.  Representing address ranges as [start, end) just generally suffers from that problem.

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PR: https://git.openjdk.java.net/jdk/pull/7343