[PATCH v3] generic: add a test for umount racing mount

[Boris Burkov <boris@bur.io>]

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 the patch:
"btrfs: fix mount failure caused by race with umount"

Signed-off-by: Boris Burkov <boris@bur.io>
---
 tests/generic/603     | 53 +++++++++++++++++++++++++++++++++++++++++++
 tests/generic/603.out |  2 ++
 tests/generic/group   |  1 +
 3 files changed, 56 insertions(+)
 create mode 100755 tests/generic/603
 create mode 100644 tests/generic/603.out

diff --git a/tests/generic/603 b/tests/generic/603
new file mode 100755
index 00000000..8e9a80e6
--- /dev/null
+++ b/tests/generic/603
@@ -0,0 +1,53 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2020 Facebook  All Rights Reserved.
+#
+# FS QA Test 603
+#
+# 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 $(seq 0 500)
+do
+	dd if=/dev/zero of="$SCRATCH_MNT/$i" bs=1M count=1 > /dev/null 2>&1
+done
+_scratch_unmount &
+_scratch_mount
+wait
+
+echo "Silence is golden"
+
+# success, all done
+status=0
+exit
diff --git a/tests/generic/603.out b/tests/generic/603.out
new file mode 100644
index 00000000..6810da89
--- /dev/null
+++ b/tests/generic/603.out
@@ -0,0 +1,2 @@
+QA output created by 603
+Silence is golden
diff --git a/tests/generic/group b/tests/generic/group
index d9ab9a31..c0ace35b 100644
--- a/tests/generic/group
+++ b/tests/generic/group
@@ -605,3 +605,4 @@
 600 auto quick quota
 601 auto quick quota
 602 auto quick encrypt
+603 auto quick
-- 
2.24.1