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

Nils Eliasson nils.eliasson at oracle.com
Fri Jan 25 13:26:13 UTC 2019

Hi Per,

Looks great!



On 2019-01-24 13:29, Per Liden wrote:
> On 1/24/19 12:05 PM, 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 
> s/stable/stale/ ;)
>> 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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