1354  * Assume that the thread A reads from the vnode vp1 into userspace
1357  *   vn_read() -> VOP_READ(vp1) -> uiomove() -> [Page Fault] ->
1359  * which establishes lock order vp1->vn_lock, then vp2->vn_lock.
1361  * backed by the pages of vnode vp1, and some page in buf2 is not
1362  * resident, we get a reversed order vp2->vn_lock, then vp1->vn_lock.
4274  * Lock pair of (possibly same) vnodes vp1, vp2, avoiding lock order
4275  * reversal.  vp1_locked indicates whether vp1 is locked; if not, vp1
4289  * If vp1 == vp2, only one, most exclusive, lock is obtained on it.
4292 vn_lock_pair(struct vnode *vp1, bool vp1_locked, int lkflags1,  in vn_lock_pair()  argument
4298 	    (vp1 == NULL && lkflags1 == 0));  in vn_lock_pair()
4304 	if (vp1 == NULL && vp2 == NULL)  in vn_lock_pair()
4307 	if (vp1 == vp2) {  in vn_lock_pair()
4315 			ASSERT_VOP_LOCKED(vp1, "vp1");  in vn_lock_pair()
4318 			if ((vp1->v_vnlock->lock_object.lo_flags &  in vn_lock_pair()
4322 			locked1 = VOP_ISLOCKED(vp1);  in vn_lock_pair()
4328 			VOP_UNLOCK(vp1);  in vn_lock_pair()
4331 		ASSERT_VOP_UNLOCKED(vp1, "vp1");  in vn_lock_pair()
4332 		vn_lock(vp1, lkflags1 | LK_RETRY);  in vn_lock_pair()
4336 	if (vp1 != NULL) {  in vn_lock_pair()
4338 		    (vp1->v_vnlock->lock_object.lo_flags & LK_NOSHARE) != 0)  in vn_lock_pair()
4340 		if (vp1_locked && VOP_ISLOCKED(vp1) != LK_EXCLUSIVE) {  in vn_lock_pair()
4341 			ASSERT_VOP_LOCKED(vp1, "vp1");  in vn_lock_pair()
4343 				VOP_UNLOCK(vp1);  in vn_lock_pair()
4344 				ASSERT_VOP_UNLOCKED(vp1,  in vn_lock_pair()
4345 				    "vp1 shared recursed");  in vn_lock_pair()
4349 			ASSERT_VOP_UNLOCKED(vp1, "vp1");  in vn_lock_pair()
4373 		vn_lock(vp1, lkflags1 | LK_RETRY);  in vn_lock_pair()
4379 			if (vp1 != NULL) {  in vn_lock_pair()
4384 				VOP_UNLOCK(vp1);  in vn_lock_pair()
4391 		if (vp2_locked && vp1 != NULL) {  in vn_lock_pair()
4393 				error = VOP_LOCK1(vp1, lkflags1 | LK_NOWAIT,  in vn_lock_pair()
4401 			vn_lock(vp1, lkflags1 | LK_RETRY);  in vn_lock_pair()
4405 	if (vp1 != NULL) {  in vn_lock_pair()
4407 			ASSERT_VOP_ELOCKED(vp1, "vp1 ret");  in vn_lock_pair()
4409 			ASSERT_VOP_LOCKED(vp1, "vp1 ret");  in vn_lock_pair()