Fix dnode allocation race

When performing concurrent object allocations using the new
multi-threaded allocator and large dnodes it's possible to
allocate overlapping large dnodes.

This case should have been handled by detecting an error
returned by dnode_hold_impl().  But that logic only checked
the returned dnp was not-NULL, and the dnp variable was not
reset to NULL when retrying.  Resolve this issue by properly
checking the return value of dnode_hold_impl().

Additionally, it was possible that dnode_hold_impl() would
misreport a dnode as free when it was in fact in use.  This
could occurs for two reasons:

* The per-slot zrl_lock must be held over the entire critical
  section which includes the alloc/free until the new dnode
  is assigned to children_dnodes.  Additionally, all of the
  zrl_lock's in the range must be held to protect moving
  dnodes.

* The dn->dn_ot_type cannot be solely relied upon to check
  the type.  When allocating a new dnode its type will be
  DMU_OT_NONE after dnode_create().  Only latter when
  dnode_allocate() is called will it transition to the new
  type.  This means there's a window when allocating where
  it can mistaken for a free dnode.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Ned Bass <bass6@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6414
Closes #6439

tests/runfiles/linux.run

@@ -364,7 +364,7 @@ tests = ['async_destroy_001_pos']
 [tests/functional/features/large_dnode]
 tests = ['large_dnode_001_pos', 'large_dnode_002_pos', 'large_dnode_003_pos',
          'large_dnode_004_neg', 'large_dnode_005_pos', 'large_dnode_006_pos',
-         'large_dnode_007_neg']
+         'large_dnode_007_neg', 'large_dnode_008_pos']
 
 [tests/functional/grow_pool]
 tests = ['grow_pool_001_pos']

tests/zfs-tests/tests/functional/features/large_dnode/Makefile.am

@@ -8,4 +8,5 @@ dist_pkgdata_SCRIPTS = \
 	large_dnode_004_neg.ksh \
 	large_dnode_005_pos.ksh \
 	large_dnode_006_pos.ksh \
-	large_dnode_007_neg.ksh
+	large_dnode_007_neg.ksh \
+	large_dnode_008_pos.ksh

tests/zfs-tests/tests/functional/features/large_dnode/large_dnode_008_pos.ksh

@@ -0,0 +1,60 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# The contents of this file are subject to the terms of the
+# Common Development and Distribution License (the "License").
+# You may not use this file except in compliance with the License.
+#
+# You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+# or http://www.opensolaris.org/os/licensing.
+# See the License for the specific language governing permissions
+# and limitations under the License.
+#
+# When distributing Covered Code, include this CDDL HEADER in each
+# file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+# If applicable, add the following below this CDDL HEADER, with the
+# fields enclosed by brackets "[]" replaced with your own identifying
+# information: Portions Copyright [yyyy] [name of copyright owner]
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2017 by Lawrence Livermore National Security, LLC.
+# Use is subject to license terms.
+#
+
+. $STF_SUITE/include/libtest.shlib
+
+#
+# DESCRIPTION:
+# Run many xattrtests on a dataset with large dnodes and xattr=sa to
+# stress concurrent allocation of large dnodes.
+#
+
+TEST_FS=$TESTPOOL/large_dnode
+
+verify_runnable "both"
+
+function cleanup
+{
+	datasetexists $TEST_FS && log_must zfs destroy $TEST_FS
+}
+
+log_onexit cleanup
+log_assert "xattrtest runs concurrently on dataset with large dnodes"
+
+log_must zfs create $TEST_FS
+log_must zfs set dnsize=auto $TEST_FS
+log_must zfs set xattr=sa $TEST_FS
+
+for ((i=0; i < 100; i++)); do
+	dir="/$TEST_FS/dir.$i"
+	log_must mkdir "$dir"
+	log_must eval "xattrtest -R -r -y -x 1 -f 1024 -k -p $dir &"
+done
+
+log_must wait
+
+log_pass