--- /dev/null
+#! /bin/bash
+# FS QA Test No. 040
+#
+# This test is motivated by an fsync issue discovered in btrfs.
+# The issue in btrfs was that adding a new hard link to an inode that already
+# had a large number of hardlinks and fsync the inode, would make the fsync
+# log replay code update the inode with a wrong link count (smaller than the
+# correct value). This resulted later in dangling directory index entries,
+# after removing most of the hard links (correct_value - wrong_value), that
+# were visible to user space but it was impossible to delete them or do
+# any other operation on them (since they pointed to an inode that didn't
+# exist anymore, resulting in -ESTALE errors).
+#
+# The btrfs issue was fixed by the following linux kernel patch:
+#
+# Btrfs: fix fsync when extend references are added to an inode
+#
+# This issue was present in btrfs since the extrefs (extend references)
+# feature was added (2012).
+#
+#-----------------------------------------------------------------------
+# Copyright (C) 2015 SUSE Linux Products GmbH. All Rights Reserved.
+# Author: Filipe Manana <fdmanana@suse.com>
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it would be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write the Free Software Foundation,
+# Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+#-----------------------------------------------------------------------
+#
+
+seq=`basename $0`
+seqres=$RESULT_DIR/$seq
+echo "QA output created by $seq"
+
+here=`pwd`
+tmp=/tmp/$$
+status=1 # failure is the default!
+
+_cleanup()
+{
+ _cleanup_flakey
+}
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/filter
+. ./common/dmflakey
+
+# real QA test starts here
+_supported_fs generic
+_supported_os Linux
+_need_to_be_root
+_require_scratch
+_require_dm_flakey
+
+rm -f $seqres.full
+
+# If the test filesystem is btrfs, make sure we create a filesystem with
+# the extend references (extrefs) feature enabled (it's enabled by default
+# in recent versions of btrfs-progs).
+if [ "$FSTYP" = "btrfs" ]; then
+ _scratch_mkfs "-O extref" >> $seqres.full 2>&1
+else
+ _scratch_mkfs >> $seqres.full 2>&1
+fi
+
+_init_flakey
+_mount_flakey
+
+# Create a test file with 3001 hard links. This number is large enough to
+# make btrfs start using extrefs at some point even if the fs has the maximum
+# possible leaf/node size (64Kb).
+echo "hello world" > $SCRATCH_MNT/foo
+for i in `seq 1 3000`; do
+ ln $SCRATCH_MNT/foo $SCRATCH_MNT/foo_link_`printf "%04d" $i`
+done
+
+# Make sure all metadata and data are durably persisted.
+sync
+
+# Add one more link to the inode that ends up being a btrfs extref and fsync
+# the inode.
+ln $SCRATCH_MNT/foo $SCRATCH_MNT/foo_link_3001
+$XFS_IO_PROG -c "fsync" $SCRATCH_MNT/foo
+
+# Simulate a crash/power loss. This makes sure the next mount
+# will see an fsync log and will replay that log.
+
+_load_flakey_table $FLAKEY_DROP_WRITES
+_unmount_flakey
+
+_load_flakey_table $FLAKEY_ALLOW_WRITES
+_mount_flakey
+
+# Now after the fsync log replay btrfs left our inode with a wrong link count N,
+# which was smaller than the correct link count M (N < M).
+# So after removing N hard links, the remaining M - N directory entries were
+# still visible to user space but it was impossible to do anything with them
+# because they pointed to an inode that didn't exist anymore. This resulted in
+# stale file handle errors (-ESTALE) when accessing those dentries for example.
+#
+# So remove all hard links except the first one and then attempt to read the
+# file, to verify we don't get an -ESTALE error when accessing the inode.
+#
+# The btrfs fsck tool also detected the incorrect inode link count and it
+# reported an error message like the following:
+#
+# root 5 inode 257 errors 2001, no inode item, link count wrong
+# unresolved ref dir 256 index 2978 namelen 13 name foo_link_2976 filetype 1 errors 4, no inode ref
+#
+# The fstests framework automatically calls fsck after a test is run, so we
+# don't need to call fsck explicitly here.
+
+echo "Link count before rm foo_link_*: $(stat --format=%h $SCRATCH_MNT/foo)"
+rm -f $SCRATCH_MNT/foo_link_*
+echo "Link count after rm foo_link_*: $(stat --format=%h $SCRATCH_MNT/foo)"
+cat $SCRATCH_MNT/foo
+
+status=0
+exit
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