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

David Holmes dholmes at openjdk.java.net
Thu Oct 28 02:55:11 UTC 2021

On Thu, 21 Oct 2021 09:14:26 GMT, Thomas Stuefe <stuefe 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)

Hi Thomas,

This seems fine to me.


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?


Marked as reviewed by dholmes (Reviewer).

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

More information about the hotspot-runtime-dev mailing list