Index: sys/geom/geom_slice.c
===================================================================
--- sys/geom/geom_slice.c
+++ sys/geom/geom_slice.c
@@ -105,6 +105,7 @@
 	struct g_provider *pp2;
 	struct g_slicer *gsp;
 	struct g_slice *gsl, *gsl2;
+	bool first_open;
 
 	gp = pp->geom;
 	cp = LIST_FIRST(&gp->consumer);
@@ -130,13 +131,35 @@
 				return (EPERM);
 		}
 	}
+
 	/* On first open, grab an extra "exclusive" bit */
-	if (cp->acr == 0 && cp->acw == 0 && cp->ace == 0)
+	first_open = false;
+	if (cp->acr == 0 && cp->acw == 0 && cp->ace == 0) {
 		de++;
+		first_open = true;
+	}
+
 	/* ... and let go of it on last close */
 	if ((cp->acr + dr) == 0 && (cp->acw + dw) == 0 && (cp->ace + de) == 1)
 		de--;
+
 	error = g_access(cp, dr, dw, de);
+
+	if (error == 0 && first_open &&
+	    (cp->acr != dr || cp->acw != dw || cp->ace != de)) {
+		/*
+		 * GEOM topology lock has been dropped and reacquired by one
+		 * of underlying geoms because it needed to perform an I/O
+		 * operation.  While the lock was not held by this thread
+		 * another thread performed a concurrent "first open" and this
+		 * thread lost the race.  So, drop the extra "exclusive" bit.
+		 */
+		g_topology_assert();
+		KASSERT(cp->ace >= 2, ("%s: lost exclusive bit", __func__));
+		g_trace(G_T_ACCESS, "%s(%s): lost first-open race", __func__,
+		    gp->name);
+		cp->ace--;
+	}
 
 	/*
 	 * Free the softc if all providers have been closed and this geom