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GC.start runs even while GC is disabled, so mmtk_handle_user_collection_request enables GC around the collection and disables it again afterward. While GC is enabled, another Ractor's GC.start can request a GC that hasn't run yet when this thread tries to re-disable GC. disable_collection fails in that state, and we panicked with "failed to re-disable GC after GC". This showed up as an intermittent crash of test_ractor_nt_switch.rb's gc_finalize_deferred test on the MMTk CI. Instead of panicking, block for the pending GC and try again. Since a GC is already requested, the retry joins it instead of starting a new one.
k0kubun
marked this pull request as ready for review
October 27, 2026 21:43
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GC.startruns even while GC is disabled, sommtk_handle_user_collection_requestenables GC around the collection and disables it again afterward. While GC is enabled, another Ractor'sGC.startcan request a GC that hasn't run yet when this thread tries to re-disable GC.disable_collectionfails in that state, and we panicked with "failed to re-disable GC after GC".This showed up as an intermittent crash of
test_ractor_nt_switch.rb'sgc_finalize_deferredtest on the MMTk CI (example). Instead of panicking, block for the pending GC and try again. Since a GC is already requested, the retry joins it instead of starting a new one.