RFR(S): 8225670: compiler/types/correctness/* tests fail with "assert(recv == __null || recv->is_klass()) failed: wrong type"
christian.hagedorn at oracle.com
Fri Aug 9 11:55:57 UTC 2019
Hi Erik, hi Tobias
Thank you for your reviews!
On 09.08.19 11:21, Erik Österlund wrote:
> Hi Christian,
> Looks good - well spotted.
> To answer your question - yes the GC (ZGC in particular, and probably
> soon Shenandoah when they hook in to the concurrent class unloading
> framework) cleans the extra data section of MDOs concurrently, under the
> extra data lock.
> However, they clear whole rows from the extra data section, under the
> extra data lock of the MDOs; they never write that the Klass is NULL. So
> I believe this bug only relates to the use of the WhiteBox API.
That was my guess, too. Thanks for clearing that up and answering the
> The row clearing of concurrent GCs synchronizes with a metadata
> preparation phase for unpacking MDOs to ciMDOs. The preparation phase
> will in a fixed-point iteration try to create ci handles for all
> encountered metadata under the extradata lock. Every time it encounters
> an uncached metadata instance, it has to release the lock due to ranking
> issues, and may also run into safepoints then. Such situations are
> detected, triggering a restart of the fixed-pont iteration.
> Once the fixed-point iteration has finished, we know that we under the
> lock walked all metadata in the extra data section without ever
> releasing the lock, have ci handles keeping all metadata alive, and
> can't have gotten any safepoints due to being in VM state. After that,
> the rows are copied and translated, and now we are guaranteed that the
> translation will always already have the ci handles cached.
Thanks for the detailed explanation!
> There is some random original copy of the raw MDO extra data that is
> performed before preparing the metadata. I don't think it is really used
> or needed. Might be interesting to remove in a future RFE. It gets
> overwritten by the subsequent row-by-row processing after metadata
I created a new RFE  and referenced this conversation.
More information about the hotspot-compiler-dev