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Index: sys/kern/vfs_subr.c
===================================================================
--- sys/kern/vfs_subr.c
+++ sys/kern/vfs_subr.c
@@ -2667,12 +2667,13 @@
* 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)
@@ -2784,6 +2785,29 @@
* 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 (vp->v_type != VCHR) {
+ 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);
+ }
+ } else {
+ if (vp->v_usecount > 0)
+ refcount_release(&vp->v_holdcnt);
+ }
if ((vp->v_iflag & VI_OWEINACT) == 0) {
oweinact = 0;
} else {
@@ -2791,8 +2815,6 @@
vp->v_iflag &= ~VI_OWEINACT;
VNODE_REFCOUNT_FENCE_REL();
}
- if (vp->v_usecount > 0)
- refcount_release(&vp->v_holdcnt);
v_incr_devcount(vp);
refcount_acquire(&vp->v_usecount);
if (oweinact && VOP_ISLOCKED(vp) == LK_EXCLUSIVE &&
@@ -2832,6 +2854,15 @@
ASSERT_VI_LOCKED(vp, __func__);
CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
+ if (vp->v_type != VCHR &&
+ 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;
+ }
if (vp->v_usecount == 0)
vholdl(vp);
if ((vp->v_iflag & VI_OWEINACT) != 0) {
@@ -2917,21 +2948,35 @@
if (func == VPUTX_VPUT)
VOP_UNLOCK(vp, 0);
- if (vp->v_type != VCHR &&
- refcount_release_if_not_last(&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.
*/
- v_decr_devcount(vp);
- if (!refcount_release(&vp->v_usecount)) {
- VI_UNLOCK(vp);
- return;
+ if (vp->v_type != VCHR) {
+ /*
+ * If we release the last usecount we take ownership of the hold
+ * count which provides liveness of the vnode, in which case we
+ * have to vdrop.
+ */
+ if (!refcount_release(&vp->v_usecount))
+ return;
+ VI_LOCK(vp);
+ /*
+ * By the time we got here someone else might have transitioned
+ * the count back to > 0.
+ */
+ if (vp->v_usecount > 0) {
+ vdropl(vp);
+ return;
+ }
+ } else {
+ VI_LOCK(vp);
+ v_decr_devcount(vp);
+ if (!refcount_release(&vp->v_usecount)) {
+ VI_UNLOCK(vp);
+ return;
+ }
}
if (vp->v_iflag & VI_DOINGINACT) {
vdropl(vp);

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