Commits on Oct 11, 2024

1. Fix a race between forwarding bits and VO bits. …

   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.

   

   wks committed Oct 11, 2024
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