vt: Do not lock request comming from terminal

only those originated by mouse.  Because the terminal surrounds
requests to vt(4) with locking.

Reported by:	bz, adrian
Reviewed by:	adrian, glebius
Approved by:	glebius (mentor)
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D57442

sys/dev/vt/vt_buf.c

@@ -45,6 +45,7 @@ static MALLOC_DEFINE(M_VTBUF, "vtbuf", "vt buffer");
 
 #define	VTBUF_LOCK(vb)		mtx_lock_spin(&(vb)->vb_lock)
 #define	VTBUF_UNLOCK(vb)	mtx_unlock_spin(&(vb)->vb_lock)
+#define	VTBUF_LOCK_OWNED(vb)	mtx_owned(&(vb)->vb_lock)
 
 #define POS_INDEX(c, r) (((r) << 12) + (c))
 #define	POS_COPY(d, s)	do {	\
@@ -740,9 +741,19 @@ vtbuf_flush_mark(struct vt_buf *vb)
 		area.tr_end.tp_col = vb->vb_scr_size.tp_col;
 		area.tr_end.tp_row = MAX(s, e) + 1;
 
-		VTBUF_LOCK(vb);
-		vtbuf_dirty(vb, &area);
-		VTBUF_UNLOCK(vb);
+		/* 
+		 * If the request originates from a keyboard, the vtbuf is
+		 * locked by teken for the entire duration of the request.
+		 * For all other sources, we avoid holding the spinlock for
+		 * extended periods.
+		 */
+		if (VTBUF_LOCK_OWNED(vb)) {
+			vtbuf_dirty(vb, &area);
+		} else {
+			VTBUF_LOCK(vb);
+			vtbuf_dirty(vb, &area);
+			VTBUF_UNLOCK(vb);
+		}
 	}
 }