RFR(M): 7009266: G1: assert(obj->is_oop_or_null(true )) failed: Error
john.cuthbertson at oracle.com
Mon Apr 4 16:43:16 PDT 2011
A new webrev for this fix can be found at:
The changes in this revision include:
* Revised barrier set calls in JNI_GetObjectField and Unsafe_getObject
so that the value that is returned is the value that gets recorded in an
SATB buffer. The code in the previous revision re-read the value of the
field potentially causing the value the is logged to be different from
* Added the static checks, suggested by Tom, in library_call.cpp.
* Removed the complicated control flow from the C1 LIR implementation of
Unsafe.getObject. Instead the checks have been moved into a code stub
while some static checks have been added.
* Re-enabled reference discovery (and therefore reference processing)
during concurrent marking as a result of pushing the changes for 7020042
On 03/28/11 10:35, John Cuthbertson wrote:
> Hi Everyone,
> A new webrev with changes based upon comments from Tom can be found
> at: http://cr.openjdk.java.net/~johnc/7009266/webrev.4/.
> The latest changes include inserting a suitably guarded barrier call
> in case the referent field of a Reference object is being read/fetched
> using JNI, reflection, or Unsafe.
> On 3/11/2011 5:34 PM, John Cuthbertson wrote:
>> Hi Everyone,
>> I'm looking for a few of volunteers to review the changes that fix
>> this assertion failure. The latest changes can be found at:
>> http://cr.openjdk.java.net/~johnc/7009266/webrev.3/ and include
>> changes based upon earlier internal reviews. The earlier changes are
>> also on cr.openjdk.java.net for reference.
>> The G1 garbage collector includes a concurrent marking algorithm that
>> makes use of snapshot-at-the-beginning or SATB. With this algorithm
>> the GC will mark all objects that are reachable at the start of
>> marking; objects that are allocated since the start of marking are
>> implicitly considered live. In order to populate the "snapshot" of
>> the object graph that existed at the start of marking, G1 employs a
>> write barrier. When an object is stored into another object's field
>> the write-barrier records the previous value of that field as it was
>> part of the "snapshot" and concurrent marking will trace the
>> sub-graph that is reachable from this previous value.
>> Unfortunately, in the presence of Reference objects, SATB might not
>> be sufficient to mark a referent object as live. Consider that, at
>> the start of marking, we have a weakly reachable object i.e. an
>> object where the only pointer to that object. If the referent is
>> obtained from the Reference object and stored to another object's
>> field (making the referent now strongly reachable and hence live) the
>> G1 write barrier will record the field's previous value but not the
>> value of the referent.
>> If the referent object is strongly reachable from some other object
>> that will be traced by concurrent marking, _or_ there is a subsequent
>> assignment to the field where we have written the referent (in which
>> case we record the previous value - the referent - in an SATB buffer)
>> then the referent will be marked live. Otherwise the referent will
>> not be marked.
>> That is the issue that was causing the failure in this CR. There was
>> a Logger object that was only reachable through a WeakReference at
>> the start of concurrent marking. During marking the Logger object is
>> obtained from the WeakReference and stored into a field of a live
>> object. The G1 write barrier recorded the previous value in the field
>> (as it is part of the snapshot at the start of marking). Since there
>> was no other assignment to the live object's field and there was no
>> other strong reference to the Logger object, the Logger object was
>> not marked. At the end of concurrent marking the Logger object was
>> considered dead and the link between the WeakReference and the Logger
>> was severed by clearing the referent field during reference processing.
>> To solve this (entirely in Hotspot and causing a performance overhead
>> for G1 only) it was decided that the best approach was to intrinsify
>> the Reference.get() method in the JIT compilers and add new
>> interpreter entry points so that the value in the referent field will
>> be recorded in an SATB buffer by the G1 pre-barrier code.
>> The changes for Zero and the C++ interpreters are place holder
>> routines but should be straight forward to implement.
>> None of the individual changes is large - they are just well
>> distributed around the JVM. :)
>> Testing: white box test; eyeballing the generated compiled and
>> interpreter code; the failing Kitchensink big-app on x86 (32/64 bit),
>> sparc (32/64 bit), Xint, Xcomp (client and server), with and without
>> G1; the GC test suite with and without G1; and jprt.
>> Thanks and regards,
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