invalid oops during oop_do() iteration?

Y. S. Ramakrishna y.s.ramakrishna at
Wed Jan 19 17:25:39 UTC 2011

Can you send a pointer to the changes that you made that led
to this issue? In my repo, I do not see a method_at() method
of JvmtiBreakpoint to try to understand what the problem is.

I didn't understand your statement "methopdOops will always be valid and
refer to the same method". (methodOops that are marked by GC as reachable
will be kept, but if you stash methodOops in yr breakpoint structure,
and these are weak pointers, as i assume they want to be, these would
be cleared (nulled) during a post-mark weak references iteration, after
which class unloading is also done.)

Not sure I understood yr problem though, so more details will help.
-- ramki

On 01/19/11 07:14, Keith McGuigan wrote:
> Hello,
> In some recent changes to JVMTI breakpoint code, I added some code to 
> update a cache during an oop_do() iteration over of methodOop.  The 
> cache update involves dereferencing the methodOop's constMethodOop and 
> getting the address to the code in it.
> Essentially, I'm doing something like this:
> JvmtiBreakpoint::oops_do(OopClosure* f) {
>   for (int i = 0; i < _num_methods; ++i) {
>     methodOop* m = method_at(i);
>     f->do_oop(m);
>   // update the cache in case the methodOop moved
>    _cache[i] =  (*m)->code_base() + _bci;
>   }
> }
> (The actual code is in GrowableCache::oops_do() in jvmtiImpl.cpp, but 
> this is the general idea)
> Doing this assumes that the methodOops will always be valid and refer to 
> the same method.  Considering what I'm seeing, I think this might be an 
> invalid assumption.   When oops_do() is called from 
> PSMarkSweep::mark_sweep_phase3(), I occasionally see that the resulting 
> methodOop* refers to something else when I'm updating the cache (not 
> sure what, but it doesn't appear to be the same methodOop).  It appears 
> to be reset correctly in a later oops_do() iteration.
> Is this expected?  And if so, is there any way for me to tell when doing 
> the oops_do() whether or not the resulting oops are valid?  We're seeing 
> assertion failures and crashes in nightly testing when the oop is not 
> the methodOop that we expected.  I'll take a quick hack to make it work 
> for now if we can fix it better later.
> If you've got anything for me to try, IM me (or email, but IM might be 
> faster) and I'll give it a shot.
> -- 
> - Keith

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