RFR: JDK-8275704: Metaspace::contains() should be threadsafe

Thomas Stuefe stuefe at openjdk.java.net
Thu Oct 28 05:27:10 UTC 2021

On Thu, 28 Oct 2021 02:51:15 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Metaspace::contains() is used in many places. It is not threadsafe since it walks the list of metaspace mappings, which can be altered concurrently. This is suspected to be the cause of JDK-8271124.
>> Currently, it does not lock, and adding a lock is not realistic either. It should work lockless.
>> This patch builds atop of https://bugs.openjdk.java.net/browse/JDK-8275582, which removed the old (pre JEP 387) technique of uncommitting metaspace memory. As a side effect, that patch changed the mapping list to an add-only structure. The only remaining place where it gets modified is in VirtualSpaceList::create_node(). Modifications are synchronized via lock. The only place where we walk the list locklessly is in Metaspace::contains(). This patch adds the appropriate memory barriers to those two places.
>> Tests:
>> - GHAs
>> - SAP nightlies (queued)
> src/hotspot/share/memory/metaspace/virtualSpaceList.hpp line 55:
>> 53: //
>> 54: // The list will only be modified under lock protection, but may be
>> 55: //  read asynchronously without lock.
> asynchronously? Did you mean concurrently?

Right, I'll correct it before pushing.


PR: https://git.openjdk.java.net/jdk/pull/6060

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