MFC r294329 (by asomers): Disallow zvol-backed ZFS pools

Using zvols as backing devices for ZFS pools is fraught with panics and
deadlocks. For example, attempting to online a missing device in the
presence of a zvol can cause a panic when vdev_geom tastes the zvol.  Better
to completely disable vdev_geom from ever opening a zvol. The solution
relies on setting a thread-local variable during vdev_geom_open, and
returning EOPNOTSUPP during zvol_open if that thread-local variable is set.

Remove the check for MUTEX_HELD(&zfsdev_state_lock) in zvol_open. Its intent
was to prevent a recursive mutex acquisition panic. However, the new check
for the thread-local variable also fixes that problem.

Also, fix a panic in vdev_geom_taste_orphan. For an unknown reason, this
function was set to panic. But it can occur that a device disappears during
tasting, and it causes no problems to ignore this departure.

1 files changed, 15 insertions, 4 deletions

diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c
index ac994f5..deb2bfd 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c
@@ -63,6 +63,13 @@ TUNABLE_INT("vfs.zfs.vdev.bio_delete_disable", &vdev_geom_bio_delete_disable);
 SYSCTL_INT(_vfs_zfs_vdev, OID_AUTO, bio_delete_disable, CTLFLAG_RW,
     &vdev_geom_bio_delete_disable, 0, "Disable BIO_DELETE");
 
+/*
+ * Thread local storage used to indicate when a thread is probing geoms
+ * for their guids.  If NULL, this thread is not tasting geoms.  If non NULL,
+ * it is looking for a replacement for the vdev_t* that is its value.
+ */
+uint_t zfs_geom_probe_vdev_key;
+
 static void
 vdev_geom_set_rotation_rate(vdev_t *vd, struct g_consumer *cp)
 { 
@@ -329,9 +336,8 @@ vdev_geom_io(struct g_consumer *cp, int cmd, void *data, off_t offset, off_t siz
 static void
 vdev_geom_taste_orphan(struct g_consumer *cp)
 {
-
-	KASSERT(1 == 0, ("%s called while tasting %s.", __func__,
-	    cp->provider->name));
+	ZFS_LOG(0, "WARNING: Orphan %s while tasting its VDev GUID.",
+	    cp->provider->name);
 }
 
 static int
@@ -578,7 +584,6 @@ vdev_geom_attach_by_guids(vdev_t *vd)
 	g_topology_assert();
 
 	zgp = g_new_geomf(&zfs_vdev_class, "zfs::vdev::taste");
-	/* This orphan function should be never called. */
 	zgp->orphan = vdev_geom_taste_orphan;
 	zcp = g_new_consumer(zgp);
 
@@ -714,6 +719,9 @@ vdev_geom_open(vdev_t *vd, uint64_t *psize, uint64_t *max_psize,
 	size_t bufsize;
 	int error;
 
+	/* Set the TLS to indicate downstack that we should not access zvols*/
+	VERIFY(tsd_set(zfs_geom_probe_vdev_key, vd) == 0);
+
 	/*
 	 * We must have a pathname, and it must be absolute.
 	 */
@@ -764,6 +772,9 @@ vdev_geom_open(vdev_t *vd, uint64_t *psize, uint64_t *max_psize,
 		}
 	}
 
+	/* Clear the TLS now that tasting is done */
+	VERIFY(tsd_set(zfs_geom_probe_vdev_key, NULL) == 0);
+
 	if (cp == NULL) {
 		ZFS_LOG(1, "Provider %s not found.", vd->vdev_path);
 		error = ENOENT;