RFR: 8212184: Incorrect oop ref strength used for referents in FinalReference (original) (raw)
Per Liden per.liden at oracle.com
Thu Nov 1 07:30:16 UTC 2018
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Thanks for reviewing, Erik and Kim!
/Per
On 10/31/2018 05:50 PM, Kim Barrett wrote:
On Oct 31, 2018, at 11:35 AM, Per Liden <per.liden at oracle.com> wrote:
Hi, After some off-line discussions with Kim and Erik about whether this should be STRONG or PHANTOM it was agreed that it technically doesn't matter. So it comes down to preference. Kim (who preferred PHANTOM) agreed to go with STRONG (which Erik and I preferred) as long there was a comment explaining why. So I updated the patch accordingly: http://cr.openjdk.java.net/~pliden/8212184/webrev.1 Looks good.
cheers, Per
On 10/16/2018 01:55 PM, Per Liden wrote: Hi Kim, On 10/15/2018 08:55 PM, Kim Barrett wrote: On Oct 15, 2018, at 10:42 AM, Per Liden <per.liden at oracle.com> wrote:
AccessBarrierSupport::resolveunknownooprefstrength() returns an incorrect oop ref strength for referents in FinalReference objects. It currently returns ONWEAKOOPREF when it should return ONSTRONGOOPREF. This is not really an issue for any GC except ZGC when using the ZHeapIterator to walk the heap while resurrection is blocked. Bug: https://bugs.openjdk.java.net/browse/JDK-8212184 Webrev: http://cr.openjdk.java.net/~pliden/8212184/webrev.0 /Per src/hotspot/share/gc/shared/accessBarrierSupport.cpp 32 if (!javalangrefReference::isreferentfield(base, offset) || 33 javalangrefReference::isfinal(base)) { 34 ds |= ONSTRONGOOPREF; This doesn't seem right. Doesn't this give the wrong answer for G1? I'm not even sure "strong" is the right answer for ZGC in the described context. What am I missing? There's no way for a mutator to get hold of the referent (except via Unsafe, which we've said we don't care about anyway). The only time we would randomly step on a referent in a Finalizer is when we're doing heap iteration in ZGC. An alternative, to perhaps make this more explicit, would be to have an ONFINALOOPREF, but it would end up doing the same thing as ONSTRONGOOPREF. /Per
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