- Make 'vm_refcnt' volatile so that compilers won't be tempted to treat

  its value as a loop invariant.  Currently this is a no-op because
  'atomic_cmpset_int()' clobbers all memory on current architectures.
- Use atomic_fetchadd_int() instead of an atomic_cmpset_int() loop to drop
  a reference in vmspace_free().

Reviewed by:	alc
MFC after:	1 month

2 files changed, 2 insertions, 6 deletions

diff --git a/sys/vm/vm_map.c b/sys/vm/vm_map.c
index 2e0f001..40c317d 100644
--- a/sys/vm/vm_map.c
+++ b/sys/vm/vm_map.c
@@ -339,15 +339,11 @@ vmspace_dofree(struct vmspace *vm)
 void
 vmspace_free(struct vmspace *vm)
 {
-	int refcnt;
 
 	if (vm->vm_refcnt == 0)
 		panic("vmspace_free: attempt to free already freed vmspace");
 
-	do
-		refcnt = vm->vm_refcnt;
-	while (!atomic_cmpset_int(&vm->vm_refcnt, refcnt, refcnt - 1));
-	if (refcnt == 1)
+	if (atomic_fetchadd_int(&vm->vm_refcnt, -1) == 1)
 		vmspace_dofree(vm);
 }
 
diff --git a/sys/vm/vm_map.h b/sys/vm/vm_map.h
index 18a1edf..8715b41 100644
--- a/sys/vm/vm_map.h
+++ b/sys/vm/vm_map.h
@@ -236,7 +236,7 @@ struct vmspace {
 	caddr_t vm_taddr;	/* (c) user virtual address of text */
 	caddr_t vm_daddr;	/* (c) user virtual address of data */
 	caddr_t vm_maxsaddr;	/* user VA at max stack growth */
-	int	vm_refcnt;	/* number of references */
+	volatile int vm_refcnt;	/* number of references */
 	/*
 	 * Keep the PMAP last, so that CPU-specific variations of that
 	 * structure on a single architecture don't result in offset