Fixed race in dtoa [Bug #17612]

Fixed the race condition when replacing `freelist` entry with its
chained next element.  At acquiring an entry, hold the entry once
with the special value, then release by replacing it with the next
element again after acquired.  If another thread is holding the
same entry at that time, spinning until the entry gets released.

Co-Authored-By: Koichi Sasada <ko1@atdot.net>

bootstraptest/test_ractor.rb

@@ -158,6 +158,17 @@
   a #
 }
 
+# dtoa race condition
+assert_equal '[:ok, :ok, :ok]', %q{
+  n = 3
+  n.times.map{
+    Ractor.new{
+      10_000.times{ rand.to_s }
+      :ok
+    }
+  }.map(&:take)
+}
+
 ###
 ###
 # Ractor still has several memory corruption so skip huge number of tests

missing/dtoa.c

@@ -526,6 +526,8 @@ typedef struct Bigint Bigint;
 
 static Bigint *freelist[Kmax+1];
 
+#define BLOCKING_BIGINT ((Bigint *)(-1))
+
 static Bigint *
 Balloc(int k)
 {
@@ -541,8 +543,10 @@ Balloc(int k)
         rv = freelist[k];
         while (rv) {
             Bigint *rvn = rv;
-            rv = ATOMIC_PTR_CAS(freelist[k], rv, rv->next);
-            if (LIKELY(rvn == rv)) {
+            rv = ATOMIC_PTR_CAS(freelist[k], rv, BLOCKING_BIGINT);
+            if (LIKELY(rv != BLOCKING_BIGINT && rvn == rv)) {
+                rvn = ATOMIC_PTR_CAS(freelist[k], BLOCKING_BIGINT, rv->next);
+                assert(rvn == BLOCKING_BIGINT);
                 ASSUME(rv);
                 break;
             }
@@ -589,7 +593,10 @@ Bfree(Bigint *v)
         }
         ACQUIRE_DTOA_LOCK(0);
         do {
-            vn = v->next = freelist[v->k];
+            do {
+                vn = ATOMIC_PTR_CAS(freelist[v->k], 0, 0);
+            } while (UNLIKELY(vn == BLOCKING_BIGINT));
+            v->next = vn;
         } while (UNLIKELY(ATOMIC_PTR_CAS(freelist[v->k], vn, v) != vn));
         FREE_DTOA_LOCK(0);
     }