Re: [PATCH -next 1/6] Revert "md: unlock mddev before reap sync_thread in action_store"

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi, Song and Guoqing

在 2023/04/06 16:53, Yu Kuai 写道:
> Hi,
> 
> 在 2023/03/29 7:58, Song Liu 写道:
>> On Wed, Mar 22, 2023 at 11:32 PM Yu Kuai <yukuai1@huaweicloud.com> wrote:
>>>
>>> Hi,
>>>
>>> 在 2023/03/23 11:50, Guoqing Jiang 写道:
>>>
>>>> Combined your debug patch with above steps. Seems you are
>>>>
>>>> 1. add delay to action_store, so it can't get lock in time.
>>>> 2. echo "want_replacement"**triggers md_check_recovery which can 
>>>> grab lock
>>>>       to start sync thread.
>>>> 3. action_store finally hold lock to clear RECOVERY_RUNNING in reap 
>>>> sync
>>>> thread.
>>>> 4. Then the new added BUG_ON is invoked since RECOVERY_RUNNING is 
>>>> cleared
>>>>       in step 3.
>>>
>>> Yes, this is exactly what I did.
>>>
>>>> sync_thread can be interrupted once MD_RECOVERY_INTR is set which means
>>>> the RUNNING
>>>> can be cleared, so I am not sure the added BUG_ON is reasonable. And
>>>> change BUG_ON
>>>
>>> I think BUG_ON() is reasonable because only md_reap_sync_thread can
>>> clear it, md_do_sync will exit quictly if MD_RECOVERY_INTR is set, but
>>> md_do_sync should not see that MD_RECOVERY_RUNNING is cleared, otherwise
>>> there is no gurantee that only one sync_thread can be in progress.
>>>
>>>> like this makes more sense to me.
>>>>
>>>> +BUG_ON(!test_bit(MD_RECOVERY_RUNNING, &mddev->recovery) &&
>>>> +!test_bit(MD_RECOVERY_INTR, &mddev->recovery));
>>>
>>> I think this can be reporduced likewise, md_check_recovery clear
>>> MD_RECOVERY_INTR, and new sync_thread triggered by echo
>>> "want_replacement" won't set this bit.
>>>
>>>>
>>>> I think there might be racy window like you described but it should be
>>>> really small, I prefer
>>>> to just add a few lines like this instead of revert and introduce new
>>>> lock to resolve the same
>>>> issue (if it is).
>>>
>>> The new lock that I add in this patchset is just try to synchronize idle
>>> and forzen from action_store（patch 3), I can drop it if you think this
>>> is not necessary.
>>>
>>> The main changes is patch 4, new lines is not much and I really don't
>>> like to add new flags unless we have to, current code is already hard
>>> to understand...
>>>
>>> By the way, I'm concerned that drop the mutex to unregister sync_thread
>>> might not be safe, since the mutex protects lots of stuff, and there
>>> might exist other implicit dependencies.
>>>
>>>>
>>>> TBH, I am reluctant to see the changes in the series, it can only be
>>>> considered
>>>> acceptable with conditions:
>>>>
>>>> 1. the previous raid456 bug can be fixed in this way too, hopefully 
>>>> Marc
>>>> or others
>>>>       can verify it.

After reading the thread:

https://lore.kernel.org/linux-raid/5ed54ffc-ce82-bf66-4eff-390cb23bc1ac@molgen.mpg.de/T/#t

The deadlock in raid456 has same conditions as raid10:
1) echo idle hold mutex to stop sync thread;
2) sync thread wait for io to complete;
3) io can't be handled by daemon thread because sb flag is set;
4) sb flag can't be cleared because daemon thread can't hold mutex;

I tried to reporduce the deadlock with the reporducer provided in the
thread, howerver, the deadlock is not reporduced after running for more
than a day.

I changed the reporducer to below:

[root@fedora raid5]# cat test_deadlock.sh
#! /bin/bash

(
         while true; do
                 echo check > /sys/block/md0/md/sync_action
                 sleep 0.5
                 echo idle > /sys/block/md0/md/sync_action
         done
) &

echo 0 > /proc/sys/vm/dirty_background_ratio
(
         while true; do
                 fio -filename=/dev/md0 -bs=4k -rw=write -numjobs=1 
-name=xxx
         done
) &

And I finially able to reporduce the deadlock with this patch
reverted(running for about an hour):

[root@fedora raid5]# ps -elf | grep " D " | grep -v grep
1 D root         156       2 16  80   0 -     0 md_wri 06:51 ? 
00:19:15 [kworker/u8:11+flush-9:0]
5 D root        2239       1  2  80   0 -   992 kthrea 06:57 pts/0 
00:02:15 sh test_deadlock.sh
1 D root       42791       2  0  80   0 -     0 raid5_ 07:45 ? 
00:00:00 [md0_resync]
5 D root       42803   42797  0  80   0 - 92175 balanc 07:45 ? 
00:00:06 fio -filename=/dev/md0 -bs=4k -rw=write -numjobs=1 -name=xxx

[root@fedora raid5]# cat /proc/2239/stack
[<0>] kthread_stop+0x96/0x2b0
[<0>] md_unregister_thread+0x5e/0xd0
[<0>] md_reap_sync_thread+0x27/0x370
[<0>] action_store+0x1fa/0x490
[<0>] md_attr_store+0xa7/0x120
[<0>] sysfs_kf_write+0x3a/0x60
[<0>] kernfs_fop_write_iter+0x144/0x2b0
[<0>] new_sync_write+0x140/0x210
[<0>] vfs_write+0x21a/0x350
[<0>] ksys_write+0x77/0x150
[<0>] __x64_sys_write+0x1d/0x30
[<0>] do_syscall_64+0x45/0x70
[<0>] entry_SYSCALL_64_after_hwframe+0x61/0xc6
[root@fedora raid5]# cat /proc/42791/stack
[<0>] raid5_get_active_stripe+0x606/0x960
[<0>] raid5_sync_request+0x508/0x570
[<0>] md_do_sync.cold+0xaa6/0xee7
[<0>] md_thread+0x266/0x280
[<0>] kthread+0x151/0x1b0
[<0>] ret_from_fork+0x1f/0x30

And with this patchset applied, I run the above reporducer for more than
a day now, and I think the deadlock in raid456 can be fixed.

Can this patchset be considered in next merge window? If so, I'll rebase
this patchset.

Thanks,
Kuai
>>>> 2. pass all the tests in mdadm
>>
>> AFAICT, this set looks like a better solution for this problem. But I 
>> agree
>> that we need to make sure it fixes the original bug. mdadm tests are not
>> in a very good shape at the moment. I will spend more time to look into
>> these tests.
> 
> While I'm working on another thread to protect md_thread with rcu, I
> found that this patch has other defects that can cause null-ptr-
> deference in theory where md_unregister_thread(&mddev->sync_thread) can
> concurrent with other context to access sync_thread, for example:
> 
> t1: md_set_readonly             t2: action_store
>                                  md_unregister_thread
>                                  // 'reconfig_mutex' is not held
> // 'reconfig_mutex' is held by caller
> if (mddev->sync_thread)
>                                   thread = *threadp
>                                   *threadp = NULL
>   wake_up_process(mddev->sync_thread->tsk)
>   // null-ptr-deference
> 
> So, I think this revert will make more sence. 😉
> 
> Thanks,
> Kuai
> 
> .
>