Clarify buffer reclamation condition

RootSet.cpp

@@ -105,12 +105,15 @@ struct RootSet::Impl {
 
   void setOldestVersion(int64_t oldestVersion) {
     this->oldestVersion = oldestVersion;
-    // This condition is basically "while it would be incorrect to read
-    // firstToFree at oldestVersion - 1"
+    // The next buffer has an entry greater than all entries in this buffer.
+    // If the next buffer does not have an entry > `oldestVersion`, then this
+    // buffer is missing an entry <= oldestVersion on the right, so it's
+    // incorrect to read this buffer at `oldestVersion` and we can safely free
+    // it.
     while (firstToFree != nullptr && firstToFree->next != nullptr &&
            firstToFree->next->versions()[firstToFree->next->end.load(
                                              std::memory_order_relaxed) -
-                                         1] < oldestVersion) {
+                                         1] <= oldestVersion) {
       auto *tmp = firstToFree;
       firstToFree = firstToFree->next;
       free(tmp);