commit 23bb9cddaef6f06521efe4f8340f992d8a0b362f
Author: Mateusz Guzik <mjg@FreeBSD.org>
Date:   Tue Jan 19 19:03:41 2021 +0100

    vfs: VIRF_MODIFY
    
    WIP

diff --git a/sys/contrib/openzfs/module/os/freebsd/zfs/zfs_znode.c b/sys/contrib/openzfs/module/os/freebsd/zfs/zfs_znode.c
index 6a21623c5f67..644692ae48fb 100644
--- a/sys/contrib/openzfs/module/os/freebsd/zfs/zfs_znode.c
+++ b/sys/contrib/openzfs/module/os/freebsd/zfs/zfs_znode.c
@@ -431,6 +431,7 @@ zfs_znode_alloc(zfsvfs_t *zfsvfs, dmu_buf_t *db, int blksz,
 		zfs_znode_free_kmem(zp);
 		return (NULL);
 	}
+	vn_irflag_set_new(vp, VIRF_MODIFY);
 	zp->z_vnode = vp;
 	vp->v_data = zp;
 
diff --git a/sys/fs/tmpfs/tmpfs_subr.c b/sys/fs/tmpfs/tmpfs_subr.c
index c74981db0f58..262714db51ec 100644
--- a/sys/fs/tmpfs/tmpfs_subr.c
+++ b/sys/fs/tmpfs/tmpfs_subr.c
@@ -671,6 +671,8 @@ tmpfs_alloc_vp(struct mount *mp, struct tmpfs_node *node, int lkflag,
 		goto unlock;
 	MPASS(vp != NULL);
 
+	vn_irflag_set_new(vp, VIRF_MODIFY);
+
 	/* lkflag is ignored, the lock is exclusive */
 	(void) vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
 
diff --git a/sys/kern/vfs_cache.c b/sys/kern/vfs_cache.c
index 5a1ebffece05..4df83e751ce6 100644
--- a/sys/kern/vfs_cache.c
+++ b/sys/kern/vfs_cache.c
@@ -4154,6 +4154,14 @@ cache_fplookup_final_modifying(struct cache_fpl *fpl)
 		return (cache_fpl_handled_error(fpl, EEXIST));
 	}
 
+	/*
+	 * Hack: UFS seems to be running into deadlocks, don't use the
+	 * feature.
+	 */
+	if ((vn_irflag_read(fpl->dvp) & VIRF_MODIFY) == 0) {
+		return (cache_fpl_partial(fpl));
+	}
+
 	/*
 	 * Secure access to dvp; check cache_fplookup_partial_setup for
 	 * reasoning.
diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
index 8461fd0d49b5..2e67dafa4424 100644
--- a/sys/kern/vfs_subr.c
+++ b/sys/kern/vfs_subr.c
@@ -6823,6 +6823,18 @@ vn_seqc_write_end_free(struct vnode *vp)
 	VNPASS(vp->v_seqc_users == 1, vp);
 }
 
+
+/*
+ * Only to be called after getnewvnode and before publishing the vnode.
+ */
+void
+vn_irflag_set_new(struct vnode *vp, short toset)
+{
+
+	VNPASS(vp->v_irflag == 0, vp);
+	atomic_store_short(&vp->v_irflag, toset);
+}
+
 void
 vn_irflag_set_locked(struct vnode *vp, short toset)
 {
diff --git a/sys/sys/vnode.h b/sys/sys/vnode.h
index 0eadfec02313..9034c44d2df9 100644
--- a/sys/sys/vnode.h
+++ b/sys/sys/vnode.h
@@ -247,6 +247,7 @@ struct xvnode {
 #define	VIRF_PGREAD	0x0002	/* Direct reads from the page cache are permitted,
 				   never cleared once set */
 #define	VIRF_MOUNTPOINT	0x0004	/* This vnode is mounted on */
+#define	VIRF_MODIFY	0x0008	/* Filesystem supports modification from LL */
 
 #define	VI_TEXT_REF	0x0001	/* Text ref grabbed use ref */
 #define	VI_MOUNT	0x0002	/* Mount in progress */
@@ -802,6 +803,7 @@ void	vn_seqc_write_end(struct vnode *vp);
 	rangelock_trywlock(&(vp)->v_rl, (start), (end), VI_MTX(vp))
 
 #define	vn_irflag_read(vp)	atomic_load_short(&(vp)->v_irflag)
+void	vn_irflag_set_new(struct vnode *vp, short toset);
 void	vn_irflag_set_locked(struct vnode *vp, short toset);
 void	vn_irflag_set(struct vnode *vp, short toset);
 void	vn_irflag_set_cond_locked(struct vnode *vp, short toset);