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Teo Camarasu pushed to branch wip/T26186 at Glasgow Haskell Compiler / GHC

Commits:

dbc45d30

by Teo Camarasu at 2025-07-11T17:54:56+01:00

rts/nonmovingGC: remove n_free

We remove the nonmovingHeap.n_free variable.

We wanted this to track the length of nonmovingHeap.free.
But this isn't possible to do atomically.

When this isn't accurate we can get a segfault by going past the end of
the list.

Instead, we just count the length of the list when we grab it in
nonmovingPruneFreeSegment.

Resolves #26186

4 changed files:

rts/sm/NonMoving.c
rts/sm/NonMoving.h
rts/sm/NonMovingAllocate.c
rts/sm/Sanity.c

Changes:

rts/sm/NonMoving.c

@@ -603,7 +603,6 @@ void nonmovingPushFreeSegment(struct NonmovingSegment *seg)
          if (cas((StgVolatilePtr) &nonmovingHeap.free, (StgWord) old, (StgWord) seg) == (StgWord) old)
              break;
+     }
 -    __sync_add_and_fetch(&nonmovingHeap.n_free, 1);
+ }
  static int
@@ -621,20 +620,26 @@ cmp_segment_ptr (const void *x, const void *y)
  void nonmovingPruneFreeSegmentList(void)
+ {
    trace(TRACE_nonmoving_gc, "Pruning free segment list.");
++
    // Atomically grab the entire free list.
    struct NonmovingSegment *free;
 -  size_t length;
    while (true) {
      free = ACQUIRE_LOAD(&nonmovingHeap.free);
 -    length = ACQUIRE_LOAD(&nonmovingHeap.n_free);
      if (cas((StgVolatilePtr) &nonmovingHeap.free,
              (StgWord) free,
              (StgWord) NULL) == (StgWord) free) {
 -        atomic_dec((StgVolatilePtr) &nonmovingHeap.n_free, length);
          break;
+     }
 -    // Save the current free list so the sanity checker can see these segments.
 -    nonmovingHeap.saved_free = free;
 +  }
 +  // Save the current free list so the sanity checker can see these segments.
 +  nonmovingHeap.saved_free = free;
++
 +  // Calculate the length of the list we've taken
 +  size_t length = 0;
 +  struct NonmovingSegment *free1 = free;
 +  while (free1) {
 +    length++;
 +    free1 = free1->link;
+   }
    // Sort the free list by address.
@@ -692,7 +697,6 @@ void nonmovingPruneFreeSegmentList(void)
        if (cas((StgVolatilePtr) &nonmovingHeap.free,
                (StgWord) rest,
                (StgWord) free) == (StgWord) rest) {
 -          __sync_add_and_fetch(&nonmovingHeap.n_free, new_length);
            break;
+       }
+     }

rts/sm/NonMoving.h

@@ -134,8 +134,6 @@ struct NonmovingHeap {
      // saved free segment list, so the sanity checker can
      // see the segments while the free list is being pruned.
      struct NonmovingSegment *saved_free;
 -    // how many segments in free segment list? accessed atomically.
 -    unsigned int n_free;
      // records the current length of the nonmovingAllocator.current arrays
      unsigned int n_caps;

rts/sm/NonMovingAllocate.c


         if (cas((StgVolatilePtr) &nonmovingHeap.free,
                 (StgWord) seg,
                 (StgWord) seg->link) == (StgWord) seg) {
-            __sync_sub_and_fetch(&nonmovingHeap.n_free, 1);
             return seg;
         }
     }

rts/sm/Sanity.c

@@ -990,7 +990,6 @@ static void checkGeneration (generation *gen,
  #endif
      if (isNonmovingGen(gen)) {
 -        ASSERT(countNonMovingSegments(nonmovingHeap.free) == (W_) nonmovingHeap.n_free * NONMOVING_SEGMENT_BLOCKS);
          ASSERT(countBlocks(nonmoving_large_objects) == n_nonmoving_large_blocks);
          ASSERT(countBlocks(nonmoving_marked_large_objects) == n_nonmoving_marked_large_blocks);

...	...	@@ -183,7 +183,6 @@ static struct NonmovingSegment *nonmovingPopFreeSegment(void)
183	183	if (cas((StgVolatilePtr) &nonmovingHeap.free,
184	184	(StgWord) seg,
185	185	(StgWord) seg->link) == (StgWord) seg) {
186		- __sync_sub_and_fetch(&nonmovingHeap.n_free, 1);
187	186	return seg;
188	187	}
189	188	}