RFR: JDK-8211279: Verify missing object equals barriers (original) (raw)

Roman Kennke rkennke at redhat.com
Mon Oct 1 12:48:40 UTC 2018

Hi Per,

GCs like Shenandoah require an extra barrier for comparing objects (oop==oop). It is easy to forget or overlook this. It would be very useful to have a flag to turn on extra verification that catches missing object equality barriers.

This change inserts an assert into == and != operators for the oop class in oopsHierarchy.hpp. This only gets compiled in fastdebug builds (when CHECKUNHANDLEDOOPS is on). It also adds a develop flag VerifyObjectEquals that is used to turn on this verification. It also adds a method oopDesc::unsafeequals(..) to be used in cases where you know what what you are doing, and really want to use direct == comparison without using barriers. This is used in e.g. ReferenceProcessor or all over the place in ShenandoahGC. The change also fixes a couple of places where oops are compared to non-oops like Universe::nonoopword() to use the oop==void* operator instead, so those don't falsely trip the verification. It doesn't make sense to turn this check on if you're not running a GC that needs it, unless you want to go fix all the oop==oop in the GC itself. Bug: https://bugs.openjdk.java.net/browse/JDK-8211279 Webrev: http://cr.openjdk.java.net/~rkennke/JDK-8211279/webrev.00/ What do you think? So this means we would have a verification option that, when enabled, always crashes the VM unless you run Shenandoah? That doesn't sound quite right to me. This should just be a noop when not using Shenandoah, don't you think?

Hmm, right. Let's add some BarrierSet-infrastructure to handle this, and remove the option (it would be a GC-'private' option). It would probably have looked slightly better to do this in BarrierSet::Access, next to the Access::equals() API, but I don't feel like adding tons of boilerplate just to add this. (Infact, this is a big red warning signal regarding the Access API...)

http://cr.openjdk.java.net/~rkennke/JDK-8211279/webrev.01/

How does this look now?

It still passes hotspot/jtreg:tier1 here.

Thanks for looking at this! Roman

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