Re: [PATCH v3 2/2] xfs: test the deadlock between the AGI and AGF with RENAME_WHITEOUT

kaixuxia
2019-09-19T12:14:23Z

On 2019/9/19 18:47, Brian Foster wrote:
> On Thu, Sep 19, 2019 at 05:08:04PM +0800, kaixuxia wrote:
>>
>>
>> On 2019/9/18 21:59, Brian Foster wrote:
>>> On Wed, Sep 18, 2019 at 07:49:22PM +0800, kaixuxia wrote:
>>>> There is ABBA deadlock bug between the AGI and AGF when performing
>>>> rename() with RENAME_WHITEOUT flag, and add this testcase to make
>>>> sure the rename() call works well.
>>>>
>>>> Signed-off-by: kaixuxia <kaixuxia@tencent.com>
>>>> ---
>>>
>>> FYI, for some reason your patch series isn't threaded on the mailing
>>> list. I thought git send-email did this by default. Assuming you're not
>>> explicitly using --no-thread, you might have to use the --thread option
>>> so this gets posted as a proper series.
>>>
>> Yeah, thanks!
>>>> tests/xfs/512 | 96 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>> tests/xfs/512.out | 2 ++
>>>> tests/xfs/group | 1 +
>>>> 3 files changed, 99 insertions(+)
>>>> create mode 100755 tests/xfs/512
>>>> create mode 100644 tests/xfs/512.out
>>>>
>>>> diff --git a/tests/xfs/512 b/tests/xfs/512
>>>> new file mode 100755
>>>> index 0000000..a2089f0
>>>> --- /dev/null
>>>> +++ b/tests/xfs/512
>>>> @@ -0,0 +1,96 @@
>>>> +#! /bin/bash
>>>> +# SPDX-License-Identifier: GPL-2.0
>>>> +# Copyright (c) 2019 Tencent. All Rights Reserved.
>>>> +#
>>>> +# FS QA Test 512
>>>> +#
>>>> +# Test the ABBA deadlock case between the AGI and AGF When performing
>>>> +# rename operation with RENAME_WHITEOUT flag.
>>>> +#
>>>> +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
>>>> +. ./common/renameat2
>>>> +
>>>> +rm -f $seqres.full
>>>> +
>>>> +# real QA test starts here
>>>> +_supported_fs xfs
>>>> +_supported_os Linux
>>>> +# single AG will cause default xfs_repair to fail. This test need a
>>>> +# single AG fs, so ignore the check.
>>>> +_require_scratch_nocheck
>>>> +_requires_renameat2 whiteout
>>>> +
>>>> +filter_enospc() {
>>>> + sed -e '/^.*No space left on device.*/d'
>>>> +}
>>>> +
>>>> +create_file()
>>>> +{
>>>> + local target_dir=$1
>>>> + local files_count=$2
>>>> +
>>>> + for i in $(seq 1 $files_count); do
>>>> + echo > $target_dir/f$i >/dev/null 2>&1 | filter_enospc
>>>> + done
>>>> +}
>>>> +
>>>> +rename_whiteout()
>>>> +{
>>>> + local target_dir=$1
>>>> + local files_count=$2
>>>> +
>>>> + # a long filename could increase the possibility that target_dp
>>>> + # allocate new blocks(acquire the AGF lock) to store the filename
>>>> + longnamepre=`$PERL_PROG -e 'print "a"x200;'`
>>>> +
>>>> + # now try to do rename with RENAME_WHITEOUT flag
>>>> + for i in $(seq 1 $files_count); do
>>>> + src/renameat2 -w $SCRATCH_MNT/f$i $target_dir/$longnamepre$i >/dev/null 2>&1
>>>> + done
>>>> +}
>>>> +
>>>> +_scratch_mkfs_xfs -d agcount=1 >> $seqres.full 2>&1 ||
>>>> + _fail "mkfs failed"
>>>
>>> This appears to be the only XFS specific bit. Could it be
>>> conditionalized using FSTYP such that this test could go under
>>> tests/generic?
>>>
>> OK, I'll move this test to tests/generic by using FSTYP.
>>
>>>> +_scratch_mount
>>>> +
>>>> +# set the rename and create file counts
>>>> +file_count=50000
>>>> +
>>>> +# create the necessary directory for create and rename operations
>>>> +createdir=$SCRATCH_MNT/createdir
>>>> +mkdir $createdir
>>>> +renamedir=$SCRATCH_MNT/renamedir
>>>> +mkdir $renamedir
>>>> +
>>>> +# create many small files for the rename with RENAME_WHITEOUT
>>>> +create_file $SCRATCH_MNT $file_count
>>>> +
>>>> +# try to create files at the same time to hit the deadlock
>>>> +rename_whiteout $renamedir $file_count &
>>>> +create_file $createdir $file_count &
>>>> +
>>>
>>> When I ran this test I noticed that the rename_whiteout task completed
>>> renaming the 50k files before the create_file task created even 30k of
>>> the 50k files. There's no risk of deadlock once one of these tasks
>>> completes, right? If so, that seems like something that could be fixed
>>> up.
>>>
>>> Beyond that though, the test itself ran for almost 19 minutes on a vm
>>> with the deadlock fix. That seems like overkill to me for a test that's
>>> so narrowly focused on a particular bug that it's unlikely to fail in
>>> the future. If we can't find a way to get this down to a reasonable time
>>> while still reproducing the deadlock, I'm kind of wondering if there's a
>>> better approach to get more rename coverage from existing tests. For
>>> example, could we add this support to fsstress and see if any of the
>>> existing stress tests might trigger the original problem? Even if we
>>> needed to add a new rename/create focused fsstress test, that might at
>>> least be general enough to provide broader coverage.
>>>
>> Yeah, rename_whiteout task run faster than create_file task, so maybe
>> we can set two different files counts for them to reduce the test run
>> time. This test ran for 380s on my vm with the fixed kernel, but we
>> still need to find a way to reduce the run time, like the 19 minutes
>> case. Actually, in most cases, the deadlock happened when the
>> rename_whiteout task completed renaming hundreds of files. 50000
>> is set just because this test take 380s on my vm which is acceptable
>> and the reproduce possibility is near 100%. So maybe we can choose a
>> proper files count to make the test runs faster. Of course, I'll
>> also try to use fsstresss and the TIME_FACTOR if they can help to
>> reduce the run time.
>>
>
> I think using different file counts as such is too unpredictable across
> different test environments. If we end up with something like the
> current test, I'd rather see explicit logic in the test to terminate the
> workload thread when the rename thread completes. This probably would
> have knocked 2-3 minutes off the slow runtime I reported above.
>
> That aside, I think the fsstress approach is preferable because there is
> at least potential to avoid the need for a new test. The relevant
> questions to me are:
>
> 1.) If you add renameat2 support to fsstress, do any of the existing
> fsstress tests reproduce the original problem?

Not sure about this, need to do research whether there are existing
fsstress tests can reproduce the problem.
>
> 2.) If not, can fsstress reproduce the problem using customized
> parameters (i.e., limited to rename and creates)? If so, we may still
> need a separate test, but it would be trivial in that it just invokes
> fsstress with particular flags for a period of time.
>
> 3.) If not, then we need to find a way for this test to run quicker. At
> this point, I'm curious how long it takes for this test to reproduce the
> problem (on a broken kernel) on average once the setup portion
> completes. More than a minute or two, for example, or tens of minutes..
> etc.?
>
About five minutes with 50000 files count on a broken kernel to reproduce
the deadlock on my vm, and the most time is preparing 50000 empty files for
the rename operation.

A example for deadlock happened when renaming 2729 files.
call trace:
root 31829 ... D+ ... /renamedir/aaaaaaaaaaaaaaaaaaa...aaaaaaaaaaaaaaa2729
# cat /proc/31829/stack
[<0>] xfs_buf_lock+0x34/0xf0 [xfs]
[<0>] xfs_buf_find+0x215/0x6c0 [xfs]
[<0>] xfs_buf_get_map+0x37/0x230 [xfs]
[<0>] xfs_buf_read_map+0x29/0x190 [xfs]
[<0>] xfs_trans_read_buf_map+0x13d/0x520 [xfs]
[<0>] xfs_read_agi+0xa8/0x160 [xfs]
[<0>] xfs_iunlink_remove+0x6f/0x2a0 [xfs]
[<0>] xfs_rename+0x57a/0xae0 [xfs]
[<0>] xfs_vn_rename+0xe4/0x150 [xfs]
[<0>] vfs_rename+0x1f4/0x7b0
[<0>] do_renameat2+0x431/0x4c0
[<0>] __x64_sys_renameat2+0x20/0x30
[<0>] do_syscall_64+0x49/0x120
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

> Brian
>
>>> Alternatively, what if this test ran a create/rename workload (on a
>>> smaller fileset) for a fixed time of a minute or two and then exited? I
>>> think it would be a reasonable compromise if the test still reproduced
>>> on some smaller frequency, it's just not clear to me how effective such
>>> a test would be without actually trying it. Maybe Eryu has additional
>>> thoughts..
>>>
>>> Brian
>>>
>>>> +wait
>>>> +echo Silence is golden
>>>> +
>>>> +# Failure comes in the form of a deadlock.
>>>> +
>>>> +# success, all done
>>>> +status=0
>>>> +exit
>>>> diff --git a/tests/xfs/512.out b/tests/xfs/512.out
>>>> new file mode 100644
>>>> index 0000000..0aabdef
>>>> --- /dev/null
>>>> +++ b/tests/xfs/512.out
>>>> @@ -0,0 +1,2 @@
>>>> +QA output created by 512
>>>> +Silence is golden
>>>> diff --git a/tests/xfs/group b/tests/xfs/group
>>>> index a7ad300..ed250d6 100644
>>>> --- a/tests/xfs/group
>>>> +++ b/tests/xfs/group
>>>> @@ -509,3 +509,4 @@
>>>> 509 auto ioctl
>>>> 510 auto ioctl quick
>>>> 511 auto quick quota
>>>> +512 auto rename
>>>> --
>>>> 1.8.3.1
>>>>
>>>> --
>>>> kaixuxia
>>
>> --
>> kaixuxia

--
kaixuxia