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Index: sys/kern/vfs_subr.c
===================================================================
--- sys/kern/vfs_subr.c
+++ sys/kern/vfs_subr.c
@@ -117,6 +117,22 @@
static void vnlru_return_batches(struct vfsops *mnt_op);
static void destroy_vpollinfo(struct vpollinfo *vi);
+/*
+ * These fences are intended for cases where some synchronization is
+ * needed between access of v_iflags and lockless vnode refcount (v_holdcnt
+ * and v_usecount) updates. Access to v_iflags is generally synchronized
+ * by the interlock, but we have some internal assertions that check vnode
+ * flags * without acquiring the lock. Thus, these fences are INVARIANTS-only
+ * for now.
+ */
+#ifdef INVARIANTS
+#define VNODE_REFCOUNT_FENCE_ACQ() atomic_thread_fence_acq()
+#define VNODE_REFCOUNT_FENCE_REL() atomic_thread_fence_rel()
+#else
+#define VNODE_REFCOUNT_FENCE_ACQ()
+#define VNODE_REFCOUNT_FENCE_REL()
+#endif
+
/*
* Number of vnodes in existence. Increased whenever getnewvnode()
* allocates a new vnode, decreased in vdropl() for VI_DOOMED vnode.
@@ -1018,6 +1034,7 @@
*/
freevnodes--;
vp->v_iflag &= ~VI_FREE;
+ VNODE_REFCOUNT_FENCE_REL();
refcount_acquire(&vp->v_holdcnt);
mtx_unlock(&vnode_free_list_mtx);
@@ -2495,6 +2512,7 @@
if (vp->v_type != VCHR &&
refcount_acquire_if_not_zero(&vp->v_usecount)) {
+ VNODE_REFCOUNT_FENCE_ACQ();
VNASSERT((vp->v_iflag & VI_OWEINACT) == 0, vp,
("vnode with usecount and VI_OWEINACT set"));
} else {
@@ -2593,6 +2611,7 @@
} else {
oweinact = 1;
vp->v_iflag &= ~VI_OWEINACT;
+ VNODE_REFCOUNT_FENCE_REL();
}
refcount_acquire(&vp->v_usecount);
v_incr_devcount(vp);
@@ -2807,6 +2826,7 @@
CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
if (!locked) {
if (refcount_acquire_if_not_zero(&vp->v_holdcnt)) {
+ VNODE_REFCOUNT_FENCE_ACQ();
VNASSERT((vp->v_iflag & VI_FREE) == 0, vp,
("_vhold: vnode with holdcnt is free"));
return;

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