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Index: sys/kern/vfs_subr.c
===================================================================
--- sys/kern/vfs_subr.c
+++ sys/kern/vfs_subr.c
@@ -2630,12 +2630,14 @@
* see doomed vnodes. If inactive processing was delayed in
* vput try to do it here.
*
- * Notes on lockless counter manipulation:
- * _vhold, vputx and other routines make various decisions based
- * on either holdcnt or usecount being 0. As long as either counter
- * is not transitioning 0->1 nor 1->0, the manipulation can be done
- * with atomic operations. Otherwise the interlock is taken covering
- * both the atomic and additional actions.
+ * Both holdcnt and usecount can be manipulated using atomics without holding any
+ * locks except in these cases which require the vnode interlock:
+ * holdcnt: 1->0 and 0->1
+ * usecount: 0->1
+ *
+ * usecount is permitted to transition 1->0 without the interlock because
+ * vnode is kept live by holdcnt.
+ *
*/
static enum vgetstate
_vget_prep(struct vnode *vp, bool interlock)
@@ -2731,6 +2733,23 @@
* Upgrade our holdcnt to a usecount.
*/
VI_LOCK(vp);
+ /*
+ * See the previous section. By the time we get here we may find
+ * ourselves in the same spot.
+ */
+ if (refcount_acquire_if_not_zero(&vp->v_usecount)) {
+#ifdef INVARIANTS
+ int old = atomic_fetchadd_int(&vp->v_holdcnt, -1) - 1;
+ VNASSERT(old > 0, vp, ("%s: wrong hold count", __func__));
+#else
+ refcount_release(&vp->v_holdcnt);
+#endif
+ VNODE_REFCOUNT_FENCE_ACQ();
+ VNASSERT((vp->v_iflag & VI_OWEINACT) == 0, vp,
+ ("%s: vnode with usecount and VI_OWEINACT set", __func__));
+ VI_UNLOCK(vp);
+ return (0);
+ }
if ((vp->v_iflag & VI_OWEINACT) == 0) {
oweinact = 0;
} else {
@@ -2738,8 +2757,6 @@
vp->v_iflag &= ~VI_OWEINACT;
VNODE_REFCOUNT_FENCE_REL();
}
- if (vp->v_usecount > 0)
- refcount_release(&vp->v_holdcnt);
refcount_acquire(&vp->v_usecount);
if (oweinact && VOP_ISLOCKED(vp) == LK_EXCLUSIVE &&
(flags & LK_NOWAIT) == 0)
@@ -2777,8 +2794,15 @@
ASSERT_VI_LOCKED(vp, __func__);
CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
- if (vp->v_usecount == 0)
- vholdl(vp);
+ if (refcount_acquire_if_not_zero(&vp->v_usecount)) {
+ VNODE_REFCOUNT_FENCE_ACQ();
+ VNASSERT(vp->v_holdcnt > 0, vp,
+ ("%s: active vnode not held", __func__));
+ VNASSERT((vp->v_iflag & VI_OWEINACT) == 0, vp,
+ ("%s: vnode with usecount and VI_OWEINACT set", __func__));
+ return;
+ }
+ vholdl(vp);
if ((vp->v_iflag & VI_OWEINACT) != 0) {
vp->v_iflag &= ~VI_OWEINACT;
VNODE_REFCOUNT_FENCE_REL();
@@ -2855,21 +2879,20 @@
if (func == VPUTX_VPUT)
VOP_UNLOCK(vp, 0);
- if (refcount_release_if_not_last(&vp->v_usecount))
+ /*
+ * If we release the last usecount we take ownership of the hold count
+ * which provides liveness of the vnode, which means we have to vdrop.
+ */
+ if (!refcount_release(&vp->v_usecount))
return;
VI_LOCK(vp);
/*
- * We want to hold the vnode until the inactive finishes to
- * prevent vgone() races. We drop the use count here and the
- * hold count below when we're done.
+ * By the time we got here someone else might have transitioned the
+ * count back to > 0. Or the vnode started undergoing inactive.
*/
- if (!refcount_release(&vp->v_usecount)) {
- VI_UNLOCK(vp);
- return;
- }
- if (vp->v_iflag & VI_DOINGINACT) {
+ if (vp->v_usecount > 0 || vp->v_iflag & VI_DOINGINACT) {
vdropl(vp);
return;
}

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