From: Yu Kuai <yukuai1@huaweicloud.com>
To: Song Liu <song@kernel.org>, Yu Kuai <yukuai1@huaweicloud.com>
Cc: Guoqing Jiang <guoqing.jiang@linux.dev>,
logang@deltatee.com, pmenzel@molgen.mpg.de, agk@redhat.com,
snitzer@kernel.org, linux-kernel@vger.kernel.org,
linux-raid@vger.kernel.org, yi.zhang@huawei.com,
yangerkun@huawei.com, Marc Smith <msmith626@gmail.com>,
Logan Gunthorpe <logang@deltatee.com>,
"yukuai (C)" <yukuai3@huawei.com>
Subject: Re: [PATCH -next 1/6] Revert "md: unlock mddev before reap sync_thread in action_store"
Date: Thu, 6 Apr 2023 16:53:04 +0800 [thread overview]
Message-ID: <9d92a862-e728-5493-52c0-abc634eb6e97@huaweicloud.com> (raw)
In-Reply-To: <CAPhsuW7c2b4yYbwNcqKW+TBL=QYEzchnVQ4pDLBT-xoBoTvQmg@mail.gmail.com>
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.
>>> 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
next prev parent reply other threads:[~2023-04-06 8:53 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-22 6:41 [PATCH -next 0/6] md: fix that MD_RECOVERY_RUNNING can be cleared while sync_thread is still running Yu Kuai
2023-03-22 6:41 ` [PATCH -next 1/6] Revert "md: unlock mddev before reap sync_thread in action_store" Yu Kuai
2023-03-22 7:19 ` Guoqing Jiang
2023-03-22 9:00 ` Yu Kuai
2023-03-22 14:32 ` Guoqing Jiang
2023-03-23 1:36 ` Yu Kuai
2023-03-23 3:50 ` Guoqing Jiang
2023-03-23 6:32 ` Yu Kuai
2023-03-28 23:58 ` Song Liu
2023-04-06 8:53 ` Yu Kuai [this message]
2023-05-05 9:05 ` Yu Kuai
2023-03-22 6:41 ` [PATCH -next 2/6] md: refactor action_store() for 'idle' and 'frozen' Yu Kuai
2023-03-22 6:41 ` [PATCH -next 3/6] md: add a mutex to synchronize idle and frozen in action_store() Yu Kuai
2023-03-22 6:41 ` [PATCH -next 4/6] md: refactor idle/frozen_sync_thread() Yu Kuai
2023-03-22 6:41 ` [PATCH -next 5/6] md: wake up 'resync_wait' at last in md_reap_sync_thread() Yu Kuai
2023-03-22 6:41 ` [PATCH -next 6/6] md: enhance checking in md_check_recovery() Yu Kuai
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