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Ben Gamari pushed to branch wip/T26053 at Glasgow Haskell Compiler / GHC

Commits:

7e7213bd

by Ben Gamari at 2025-08-19T10:21:24-04:00

nonmoving: Use get_itbl instead of explicit loads

This is cleaner and also fixes unnecessary (and unsound) use of
`volatile`.

a84b4bae

by Ben Gamari at 2025-08-19T10:21:24-04:00

rts/Scav: Handle WHITEHOLEs in scavenge_one

`scavenge_one`, used to scavenge mutable list entries, may encounter
`WHITEHOLE`s when the non-moving GC is in use via two paths:

  1. when an MVAR is being marked concurrently
  2. when the object belongs to a chain of selectors being short-cutted.

Fixes #26204.

2 changed files:

rts/sm/NonMovingMark.c
rts/sm/Scav.c

Changes:

rts/sm/NonMovingMark.c

@@ -619,7 +619,7 @@ inline void updateRemembSetPushThunk(Capability *cap, StgThunk *thunk)
+ {
      const StgInfoTable *info;
      do {
 -        info = *(StgInfoTable* volatile*) &thunk->header.info;
 +        info = (StgInfoTable*) RELAXED_LOAD(&thunk->header.info);
      } while (info == &stg_WHITEHOLE_info);
      const StgThunkInfoTable *thunk_info = THUNK_INFO_PTR_TO_STRUCT(info);
@@ -1343,7 +1343,7 @@ mark_closure (MarkQueue *queue, const StgClosure *p0, StgClosure **origin)
              goto done;
          case WHITEHOLE:
 -            while (*(StgInfoTable* volatile*) &p->header.info == &stg_WHITEHOLE_info)
 +            while (RELAXED_LOAD(&p->header.info) == &stg_WHITEHOLE_info)
  #if defined(PARALLEL_GC)
                  busy_wait_nop()
  #endif
@@ -1714,7 +1714,7 @@ mark_closure (MarkQueue *queue, const StgClosure *p0, StgClosure **origin)
          break;
      case WHITEHOLE:
 -        while (*(StgInfoTable* volatile*) &p->header.info == &stg_WHITEHOLE_info);
 +        while ((StgInfoTable *) RELAXED_LOAD(&p->header.info) == &stg_WHITEHOLE_info);
          goto try_again;
      case COMPACT_NFDATA:

rts/sm/Scav.c

@@ -1276,6 +1276,7 @@ scavenge_one(StgPtr p)
      bool no_luck;
      bool saved_eager_promotion;
 +try_again:
      saved_eager_promotion = gct->eager_promotion;
      ASSERT(LOOKS_LIKE_CLOSURE_PTR(p));
@@ -1617,6 +1618,13 @@ scavenge_one(StgPtr p)
          scavenge_continuation((StgContinuation *)p);
          break;
 +    case WHITEHOLE:
 +        // This may happen when the nonmoving GC is in use via two paths:
 +        //   1. when an MVAR is being marked concurrently
 +        //   2. when the object belongs to a chain of selectors being short-cutted.
 +        while (get_itbl((StgClosure*) p) == &stg_WHITEHOLE_info);
 +        goto try_again;
++
      default:
          barf("scavenge_one: strange object %d", (int)(info->type));
+     }