compare-and-swap for installing page tables

1b93142d · Nickolai Zeldovich · 39b1cf6b · 1b93142d
--- a/vm.c
+++ b/vm.c
@@ -62,9 +62,11 @@ walkpgdir(pde_t *pgdir, const void *va, int create)
  pde_t *pde;
  pte_t *pgtab;

+ retry:
  pde = &pgdir[PDX(va)];
-  if(*pde & PTE_P){
-    pgtab = (pte_t*)PTE_ADDR(*pde);
+  pde_t pdev = *pde;
+  if(pdev & PTE_P){
+    pgtab = (pte_t*)PTE_ADDR(pdev);
  } else {
    if(!create || (pgtab = (pte_t*)kalloc()) == 0)
      return 0;
@@ -73,7 +75,10 @@ walkpgdir(pde_t *pgdir, const void *va, int create)
    // The permissions here are overly generous, but they can
    // be further restricted by the permissions in the page table 
    // entries, if necessary.
-    *pde = PADDR(pgtab) | PTE_P | PTE_W | PTE_U;
+    if (!__sync_bool_compare_and_swap(pde, pdev, PADDR(pgtab) | PTE_P | PTE_W | PTE_U)) {
+      kfree((void*) pgtab);
+      goto retry;
+    }
  }
  return &pgtab[PTX(va)];
 }