Fix race between verify_pool and the marking phase

`verify_pool` is a function used to check the invariant that a major
pool should contain no garbage after sweeping. It is obviously allowed
to race with marking (which can only turn UNMARKED objects into MARKED
or NOT_MARKABLE, but not into GARBAGE), we only need to mark the read as
a relaxed atomic read.

.github/workflows/tsan.yml

@@ -64,11 +64,9 @@ jobs:
           - id: normal
             name: normal
             dependencies: libunwind-dev
-          # Running with the debug runtime is disabled until the data races in
-          # it are fixed (see #12902).
-          #- id: debug
-          #  name: debug runtime
-          #  dependencies: libunwind-dev
+          - id: debug
+            name: debug runtime
+            dependencies: libunwind-dev
     steps:
       - name: Download Artifact
         uses: actions/download-artifact@v3

runtime/shared_heap.c

@@ -1320,7 +1320,7 @@ static void verify_pool(pool* a, sizeclass sz, struct mem_stats* s) {
     s->overhead += POOL_SLAB_WOFFSET(sz);
 
     while (p + wh <= end) {
-      header_t hd = (header_t)*p;
+      header_t hd = atomic_load_relaxed(p);
       CAMLassert(hd == 0 || !Has_status_hd(hd, caml_global_heap_state.GARBAGE));
       if (hd) {
         s->live += Whsize_hd(hd);