RFR(M): 7009266: G1: assert(obj->is_oop_or_null(true )) failed: Error
john.cuthbertson at oracle.com
Mon Mar 28 10:35:28 PDT 2011
A new webrev with changes based upon comments from Tom can be found at:
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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