Bug #22509: MMTk: GC.start panics with "failed to re-disable GC after GC" while a Ractor is being created

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 4.1.0dev (2026-11-03T16:41:26Z master a8b095a07e) +PRISM +GC[mmtk] [x86_64-linux]
• Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN

[hsbt (Hiroshi SHIBATA)]

The mmtk jobs of the ModGC workflow fail intermittently at bootstraptest/test_ractor_nt_switch.rb:190, where 8 Ractors call GC.start repeatedly. Over the 7 days from 2026-10-29 to 11-04, 21 of 376 ModGC run attempts (5.6%) failed this way, about a third of the attempts where an mmtk job failed. On master it was 5 of 120 pushes (4.2%). The ubuntu job hits it more often, 18 of 362 jobs (5.0%) against 3 of 335 on macOS (0.9%).

thread '<unnamed>' (62192) panicked at src/api.rs:311:33:
failed to re-disable GC after GC
   2: mmtk_handle_user_collection_request
   3: gc_start_internal
             at ../src/gc.c:4370:5
<internal:gc>:66: [BUG] Ruby mutator thread panicked

MMTk has one process-wide switch for collection, and rb_gc_impl_gc_disable in gc/mmtk/mmtk.c flips it whatever objspace it is given. Since per-Ractor GC (Feature #22227), Ractor.new disables GC for the child's objspace while it allocates the child's main thread, so with MMTk that turns off collection for every Ractor.

When GC is disabled, mmtk_handle_user_collection_request enables collection, collects, and disables it again. If another Ractor has requested a GC in between, mmtk-core's disable_collection returns Err and the binding panics, as the TODO above it already notes. In 20 of the 21 crashes the bug report lists 4 to 8 Ractors, so the parent was still creating children.

From reading the code, if the parent re-enables GC before that re-disable, collection instead stays off with nothing left to enable it. I have not confirmed this.

Failing jobs on master:
https://github.com/ruby/ruby/actions/runs/41676768677/job/126059382824
https://github.com/ruby/ruby/actions/runs/41615058020/job/125846322767
https://github.com/ruby/ruby/actions/runs/41522777008/job/125542736708
https://github.com/ruby/ruby/actions/runs/41042731751/job/124019235630
https://github.com/ruby/ruby/actions/runs/40952873030/job/123725292165

Code:
https://github.com/ruby/ruby/blob/65b5afc46740bc7a3cb0e085f68a371024d1ca3e/gc/mmtk/src/api.rs#L290-L313
https://github.com/ruby/ruby/blob/65b5afc46740bc7a3cb0e085f68a371024d1ca3e/gc/mmtk/mmtk.c#L827-L837
https://github.com/ruby/ruby/blob/65b5afc46740bc7a3cb0e085f68a371024d1ca3e/thread.c#L1159-L1169
https://github.com/mmtk/mmtk-core/blob/42c1ac176c4d931c5503e880e650bb9e84bf802c/src/mmtk.rs#L396-L407

[peterzhu2118 (Peter Zhu)]

• Status changed from Open to Closed

I think this should fix it: https://github.com/ruby/mmtk/pull/134

[hsbt (Hiroshi SHIBATA)]

Thanks @k0kubun (Takashi Kokubun) and @peterzhu2118 (Peter Zhu)