RFR: 8217717: ZGC: Broken oop map in C1 load barrier stub

Erik Österlund erik.osterlund at oracle.com
Thu Jan 24 13:49:32 UTC 2019

Hi Per,

Looks good.


On 2019-01-24 12:05, Per Liden wrote:
> The C1 load barrier stub for ZGC can emit a patching area for a lea 
> instruction. However, the call into the VM to do the patching at this 
> site is not associated with a proper oop map. As a result, we can end 
> up with a stable oop in the base pointer register if a GC safepoint 
> happens when we enter the VM to patch.
> This problem is not super easy to reproduce by default, but is a 
> serious bug when it happens. However, it is fairly easy to reproduce 
> when also injecting a small delay in the 
> ZNMethodTable::register_nmethod().
> The proposed solution is to never allow patching to happen inside the 
> load barrier stub. Instead, if patching is needed, we emit the lea 
> instruction as part of the load preceding the load barrier. That will 
> automatically generate the proper oop map for the patch site.
> Bug: https://bugs.openjdk.java.net/browse/JDK-8217717
> Webrev: http://cr.openjdk.java.net/~pliden/8217717/webrev.0
> Testing: Manual testing and inspection of generated code. Passes 
> gc-test-suite. Currently running tier1-3 on all platforms, and tier1-7 
> on Linux/x86.
> /Per

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