use versioned ptrs to avoid aba in kmalloc too

b42c4c52 · Nickolai Zeldovich · 883a02e7 · b42c4c52
--- a/kernel/kmalloc.cc
+++ b/kernel/kmalloc.cc
@@ -19,7 +19,7 @@ struct header {
 };
 struct freelist {
-  std::atomic<header*> buckets[KMMAX+1];
+  versioned<header*> buckets[KMMAX+1];
  char name[MAXNAME];
 };
@@ -46,9 +46,12 @@ morecore(int c, int b)
  assert(sz >= sizeof(header));
  for(char *q = p; q + sz <= p + PGSIZE; q += sz){
    struct header *h = (struct header *) q;
-    h->next = freelists[c].buckets[b];
+    for (;;) {
-    while (!cmpxch_update(&freelists[c].buckets[b], &h->next, h))
+      auto headptr = freelists[c].buckets[b].load();
-      ; /* spin */
+      h->next = headptr.ptr();
+      if (freelists[c].buckets[b].compare_exchange(headptr, h))
+        break;
+    }
  }
  return 0;
@@ -81,7 +84,8 @@ kmalloc(u64 nbytes)
  int c = mycpu()->id;
  for (;;) {
-    h = freelists[c].buckets[b];
+    auto headptr = freelists[c].buckets[b].load();
+    h = headptr.ptr();
    if (!h) {
      if (morecore(c, b) < 0) {
        cprintf("kmalloc(%d) failed\n", (int) nbytes);
@@ -89,7 +93,7 @@ kmalloc(u64 nbytes)
      }
    } else {
      header *nxt = h->next;
-      if (cmpxch(&freelists[c].buckets[b], h, nxt)) {
+      if (freelists[c].buckets[b].compare_exchange(headptr, nxt)) {
        if (h->next != nxt)
          panic("kmalloc: aba race");
        break;
@@ -117,9 +121,12 @@ kmfree(void *ap, u64 nbytes)
    memset(ap, 3, (1<<b));
  int c = mycpu()->id;
-  h->next = freelists[c].buckets[b];
+  for (;;) {
-  while (!cmpxch_update(&freelists[c].buckets[b], &h->next, h))
+    auto headptr = freelists[c].buckets[b].load();
-    ; /* spin */
+    h->next = headptr.ptr();
+    if (freelists[c].buckets[b].compare_exchange(headptr, h))
+      break;
+  }
  mtunlabel(mtrace_label_heap, ap);
 }