Fix a race between forwarding bits and VO bits. #1214
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k-sareen
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Oct 11, 2024
The current code sets the forwarding bits before setting the VO bit when copying an object. If another GC worker is attempting to forward the same object, it may observe the forwarding bits being `FORWARDED` but the VO bit is not set. This violates the semantics of VO bits because VO bits should be set for both from-space and to-space copies. This will affect VM bindings that assert slots always refer to a valid object when scanning objects and may update the same slot multiple times for some reasons. This revision provides a mechanism to ensure that all necessary metadata are set before setting forwarding bits to `FORWARDED`. Currently it affects the VO bits and the mark bits (which are used to update the VO bits in Immix-based plans). It may be used for other metadata introduced in the future.
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Updated mmtk-core to fix a race that caused GC workers to sometimes observe copied objects not having VO bits. See: mmtk/mmtk-core#1214 The ruby repo is merged with the official master branch. Fixes: mmtk#106
This was referenced Oct 11, 2024
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The current code sets the forwarding bits before setting the VO bit when copying an object. If another GC worker is attempting to forward the same object, it may observe the forwarding bits being
FORWARDEDbut the VO bit is not set. This violates the semantics of VO bits because VO bits should be set for both from-space and to-space copies. This will affect VM bindings that assert slots always refer to a valid object when scanning objects and may update the same slot multiple times for some reasons.This revision provides a mechanism to ensure that all necessary metadata are set before setting forwarding bits to
FORWARDED. Currently it affects the VO bits and the mark bits (which are used to update the VO bits in Immix-based plans). It may be used for other metadata introduced in the future.