Crash using sun.misc.Unsafe.static
forax at univ-mlv.fr
Tue Jul 17 16:39:46 PDT 2012
On 07/18/2012 12:55 AM, John Rose wrote:
> On Jul 16, 2012, at 4:06 PM, Krystal Mok wrote:
>> And you're right that this has to do with the PermGen removal
>> project. The difference comes from , which is a part of CR 7017732.
>> To be specific, before the 7017732, static fields are stored in the
>> instanceKlass of a Java class; an instanceKlass (or its enclosing
>> klassOopDesc to be exact) is not an Java object.
>> After 7017732, static fields are moved to the tail of java.lang.Class
>> instances, which are Java objects.
>> So to answer your question, you just shouldn't expect the the code to
>> work in JDK6/HotSpot.
> The API for unsafe access to statics is designed to allow
> implementations to do one of two things:
> 1. store a static variable at a fixed offset within a managed object
> (addressable via Java references)
> 2. store a static variable at an arbitrary but fixed 64-bit VA (in
> which case the object base is just null)
> The JDK 6 system is in a middle state, where the instanceKlass is a
> managed object and can move is not compatible with Java references.
> I think there was a time when non-Java oops could be manipulated via
> Java reference variables, but the practice has always been rather… unsafe.
> This could be fixed in JDK 6, but it's probably not worth it. I
> recommend spinning bytecoded accessors for statics on JDK 6. On JDK 7
> and later, the Class is the base (as Kris pointed out). This is
> probably how it's going to stay.
I spin bytecode by default if the field is visible from the callsite and
wanted to use unsafe with a constant base object
if the field is not visible. So to workaround that bug, I store the base
object in a non-constant field exactly like reflection does, so it works
on jdk6. Exactly, I should try to detect if the VM is hotspot or not
because neither JRockit nor J9 have the same issue.
> Thanks for the report.
> — John
> P.S. For multi-tenancy VMs, the addressing arithmetic for statics
> would need to take into account the task ID. But the above design
> (baseOrNull + longOffset) still works, since the unsafe API doesn't
> say how the two components get combined.
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