D21411.diff
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	Index: head/sys/kern/vfs_default.c
	===================================================================
	--- head/sys/kern/vfs_default.c
	+++ head/sys/kern/vfs_default.c
	@@ -588,22 +588,28 @@
	} / ap;
	{
	struct mount *mp;
	+ struct vnode *vp;

	/*
	- * XXX Since this is called unlocked we may be recycled while
	- * attempting to ref the mount. If this is the case or mountpoint
	- * will be set to NULL. We only have to prevent this call from
	- * returning with a ref to an incorrect mountpoint. It is not
	- * harmful to return with a ref to our previous mountpoint.
	+ * Note that having a reference does not prevent forced unmount from
	+ * setting ->v_mount to NULL after the lock gets released. This is of
	+ * no consequence for typical consumers (most notably vn_start_write)
	+ * since in this case the vnode is VI_DOOMED. Unmount might have
	+ * progressed far enough that its completion is only delayed by the
	+ * reference obtained here. The consumer only needs to concern itself
	+ * with releasing it.
	*/
	- mp = ap->a_vp->v_mount;
	- if (mp != NULL) {
	- vfs_ref(mp);
	- if (mp != ap->a_vp->v_mount) {
	- vfs_rel(mp);
	- mp = NULL;
	- }
	+ vp = ap->a_vp;
	+ mp = vp->v_mount;
	+ MNT_ILOCK(mp);
	+ if (mp != vp->v_mount) {
	+ MNT_IUNLOCK(mp);
	+ mp = NULL;
	+ goto out;
	}
	+ MNT_REF(mp);
	+ MNT_IUNLOCK(mp);
	+out:
	*(ap->a_mpp) = mp;
	return (0);
	}