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

Zhengyu Gu zgu at openjdk.java.net
Wed Oct 27 13:10:12 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)

Changes requested by zgu (Reviewer).

src/hotspot/share/memory/metaspace/virtualSpaceList.cpp line 193:

> 191: bool VirtualSpaceList::contains(const MetaWord* p) const {
> 192:   // Note: needs to work without locks.
> 193:   const VirtualSpaceNode* vsn = Atomic::load_acquire(&_first_node);

I believe vsn->next() now also requires load_acquire semantics.

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


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