generic: add a test for umount racing mount

Test if dirtying many inodes (which can delay umount) then
unmounting and quickly mounting again causes the mount to fail.

A race, which breaks the test in btrfs, is fixed by kernel commit
48cfa61b58a1 ("btrfs: fix mount failure caused by race with umount")

Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/generic/604

@@ -0,0 +1,52 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2020 Facebook  All Rights Reserved.
+#
+# FS QA Test 604
+#
+# Evicting dirty inodes can take a long time during umount.
+# Check that a new mount racing with such a delayed umount succeeds.
+#
+seq=`basename $0`
+seqres=$RESULT_DIR/$seq
+echo "QA output created by $seq"
+
+here=`pwd`
+tmp=/tmp/$$
+status=1	# failure is the default!
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+_cleanup()
+{
+	cd /
+	rm -f $tmp.*
+}
+
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/filter
+
+# remove previous $seqres.full before test
+rm -f $seqres.full
+
+# real QA test starts here
+
+# Modify as appropriate.
+_supported_fs generic
+_supported_os Linux
+_require_scratch
+
+_scratch_mkfs > /dev/null 2>&1
+_scratch_mount
+for i in $(seq 0 500); do
+	$XFS_IO_PROG -c "pwrite 0 4K" $SCRATCH_MNT/$i >/dev/null 2>&1
+done
+_scratch_unmount &
+_scratch_mount
+wait
+
+echo "Silence is golden"
+
+# success, all done
+status=0
+exit

tests/generic/604.out

@@ -0,0 +1,2 @@
+QA output created by 604
+Silence is golden

tests/generic/group

@@ -606,3 +606,4 @@
 601 auto quick quota
 602 auto quick encrypt
 603 auto quick quota
+604 auto quick mount