RFR(L) 8153224 Monitor deflation prolong safepoints (CR11/v2.11/14-for-jdk15)

[Daniel D. Daugherty]

On 5/15/20 9:17 AM, Erik Österlund wrote:
> Replying to myself here.
>
> On 2020-05-15 13:07, Erik Österlund wrote:
>> Oh and we probably need a normal loadload() between reading _owner 
>> and _contentions in the is_async_deflating function. The
>> loads re-ordering implies we could spuriously read an old value for 
>> _owner (not deflating) and a new value for _contentions, leading it
>> to believe it isn't deflating even though it is. But that is easier 
>> to reason about because the loads are reading values written by the 
>> same thread.
>>
>
> On that note, why does is_async_deflating() check both the _owner and 
> _contentions? It requires both
> to be updated to indicate deflation. But it seems to me that checking 
> only _contentions is enough.
>
> My understanding of the algorithm that making _contentions negative is 
> the linearization point at which
> we have decided that the monitor is async deflating and should no 
> longer be used. The deflation marker
> in the owner field is just the thing we poke in there before reaching 
> the linearization point to force fast paths
> that normally don't check _contentions to take a slow path instead, 
> that checks it.
>
> If _contentions < 0, then it is implied that _owner == deflating (and 
> we assert that in some places).
> It seems to me that the only thing checking the owner does here is to 
> cause headache and spurious false
> negatives due to requiring the careful fencing I described in order to 
> return precisely the same result we
> would get by looking only if the _contentions counter is negative. Or 
> am I missing something? Is there a
> valid case where the _contentions is negative, yet the _owner is not 
> deflating, that we need to catch
> and distinguish? I think not, and I worry about some other asserts if 
> there truly is.

You have forgotten about the A-B-A race where a racing thread can enter
an ObjectMonitor after the deflater thread sets the owner field and before
the deflater thread can change contentions.

There's a whole subsection on it in the wiki:

https://wiki.openjdk.java.net/display/HotSpot/Async+Monitor+Deflation#AsyncMonitorDeflation-T-enterWinsByA-B-A

Dan


>
> Thanks,
> /Erik