linux-kernel - Re: [PATCH v5 01/14] md: don't ignore suspended array in md_check

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Date: Sun, 18 Feb 2024 11:24:34 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Xiao Ni <xni@...hat.com>, Yu Kuai <yukuai1@...weicloud.com>
Cc: mpatocka@...hat.com, heinzm@...hat.com, blazej.kucman@...ux.intel.com,
 agk@...hat.com, snitzer@...nel.org, dm-devel@...ts.linux.dev,
 song@...nel.org, jbrassow@....redhat.com, neilb@...e.de, shli@...com,
 akpm@...l.org, linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
 yi.zhang@...wei.com, yangerkun@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v5 01/14] md: don't ignore suspended array in
 md_check_recovery()

Hi,

在 2024/02/18 11:15, Xiao Ni 写道:
> On Sun, Feb 18, 2024 at 10:34 AM Yu Kuai <yukuai1@...weicloud.com> wrote:
>>
>> Hi,
>>
>> 在 2024/02/18 10:27, Xiao Ni 写道:
>>> On Sun, Feb 18, 2024 at 9:46 AM Yu Kuai <yukuai1@...weicloud.com> wrote:
>>>>
>>>> Hi,
>>>>
>>>> 在 2024/02/18 9:33, Xiao Ni 写道:
>>>>> The deadlock problem mentioned in this patch should not be right?
>>>>
>>>> No, I think it's right. Looks like you are expecting other problems,
>>>> like mentioned in patch 6, to be fixed by this patch.
>>>
>>> Hi Kuai
>>>
>>> Could you explain why step1 and step2 from this comment can happen
>>> simultaneously? From the log, the process should be
>>> The process is :
>>> dev_remove->dm_destroy->__dm_destroy->dm_table_postsuspend_targets(raid_postsuspend)
>>> -> dm_table_destroy(raid_dtr).
>>> After suspending the array, it calls raid_dtr. So these two functions
>>> can't happen simultaneously.
>>
>> You're removing the target directly, however, dm can suspend the disk
>> directly, you can simplily:
>>
>> 1) dmsetup suspend xxx
>> 2) dmsetup remove xxx
> 
> For dm-raid, the design of suspend stops sync thread first and then it
> calls mddev_suspend to suspend array. So I'm curious why the sync
> thread can still exit when array is suspended. I know the reason now.
> Because before f52f5c71f (md: fix stopping sync thread), the process
> is raid_postsuspend->md_stop_writes->__md_stop_writes
> (__md_stop_writes sets MD_RECOVERY_FROZEN). In patch f52f5c71f, it
> doesn't set MD_RECOVERY_FROZEN in __md_stop_writes anymore.
> 
> The process changes to
> 1. raid_postsuspend->md_stop_writes->__md_stop_writes->stop_sync_thread
> (wait until MD_RECOVERY_RUNNING clears)
> 2. md thread -> md_check_recovery -> unregister_sync_thread ->
> md_reap_sync_thread (clears MD_RECOVERY_RUNNING, stop_sync_thread
> returns, md_reap_sync_thread sets MD_RECOVERY_NEEDED)
> 3. raid_postsuspend->mddev_suspend
> 4. md sync thread starts again because __md_stop_writes doesn't set
> MD_RECOVERY_FROZEN.
> It's the reason why we can see sync thread still happens when raid is suspended.
> 
> So the patch fix this problem should:

As I said, this is really a different problem from this patch, and it is
fixed seperately by patch 9. Please take a look at that patch.

Thanks,
Kuai

> 
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 9e41a9aaba8b..666761466f02 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -6315,6 +6315,7 @@ static void md_clean(struct mddev *mddev)
> 
>   static void __md_stop_writes(struct mddev *mddev)
>   {
> +       set_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
>          stop_sync_thread(mddev, true, false);
>          del_timer_sync(&mddev->safemode_timer);
> 
> Like other places which call stop_sync_thread, it needs to set the
> MD_RECOVERY_FROZEN bit.
> 
> Regards
> Xiao
> 
>>
>> Please also take a look at other patches, why step 1) can't stop sync
>> thread.
>>
>> Thanks,
>> Kuai
>>
>>>
>>>
>>>>
>>>> Noted that this patch just fix one case that MD_RECOVERY_RUNNING can't
>>>> be cleared, I you are testing this patch alone, please make sure that
>>>> you still triggered the exactly same case:
>>>>
>>>> - MD_RCOVERY_RUNNING can't be cleared while array is suspended.
>>>
>>> I'm not testing this patch. I want to understand the patch well. So I
>>> need to understand the issue first. I can't understand how this
>>> deadlock (step1,step2) happens.
>>>
>>> Regards
>>> Xiao
>>>>
>>>> Thanks,
>>>> Kuai
>>>>
>>>
>>> .
>>>
>>
> 
> .
>