Attachment 190802 Details for Bug 225960 – possible solution

[patch] possible solution

g_slice_access-race-detection.diff (text/plain), 1.56 KB, created by Andriy Gapon on 2018-02-19 15:00:01 UTC

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Creator: Andriy Gapon

Created: 2018-02-19 15:00:01 UTC

Size: 1.56 KB

patch

obsolete

>diff --git a/sys/geom/geom_slice.c b/sys/geom/geom_slice.c
>index 6cf17fcdfde3a..4d8b5088ce5d0 100644
>--- a/sys/geom/geom_slice.c
>+++ b/sys/geom/geom_slice.c
>@@ -103,6 +103,7 @@ g_slice_access(struct g_provider *pp, int dr, int dw, int de)
> 	struct g_provider *pp2;
> 	struct g_slicer *gsp;
> 	struct g_slice *gsl, *gsl2;
>+	bool first_open;
> 
> 	gp = pp->geom;
> 	cp = LIST_FIRST(&gp->consumer);
>@@ -128,14 +129,36 @@ g_slice_access(struct g_provider *pp, int dr, int dw, int de)
> 				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
> 	 * is being removed.

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Attachments on bug 225960: 190802