D14458.id.diff
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	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