Use cmpswap in the amd64 page table code.

554c58e9 · Silas Boyd-Wickizer · 36f95831 · 554c58e9 · 554c58e9
--- a/kernel.h
+++ b/kernel.h
@@ -13,6 +13,8 @@ static inline void *p2v(uptr a) { return (void *) a + KBASE; }
 #define NELEM(x) (sizeof(x)/sizeof((x)[0]))
+#define cmpswap(ptr, old, new) __sync_bool_compare_and_swap(ptr, old, new)
 struct spinlock;
 struct condvar;
 struct context;

--- a/vm.c
+++ b/vm.c
@@ -25,17 +25,21 @@ descend(pme_t *dir, void *va, u64 flags, int create, int level)
  pme_t entry;
  pme_t *next;
+retry:
  dir = &dir[PX(level, va)];
  entry = *dir;
-  if (entry == 0) {
+  if (entry & PTE_P) {
+    next = p2v(PTE_ADDR(entry));
+  } else {
    if (!create)
      return NULL;
-    next = (u64*) pgalloc();
+    next = (pme_t*) pgalloc();
    if (!next)
      return NULL;
-    *dir = v2p(next) | PTE_P | PTE_W | flags;
+    if (!cmpswap(dir, entry, v2p(next) | PTE_P | PTE_W | flags)) {
-  } else {
+      kfree((void*) next);
-    next = p2v(PTE_ADDR(entry));
+      goto retry;
+    }
  }
  return next;