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Subject: Re: [dm-devel] [PATCH -next v2 3/6] md: add a mutex to synchronize
 idle and frozen in action_store()
To:     Xiao Ni <xni@redhat.com>, Yu Kuai <yukuai1@huaweicloud.com>,
        guoqing.jiang@linux.dev, agk@redhat.com, snitzer@kernel.org,
        dm-devel@redhat.com, song@kernel.org
Cc:     yi.zhang@huawei.com, yangerkun@huawei.com,
        linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20230529132037.2124527-1-yukuai1@huaweicloud.com>
 <20230529132037.2124527-4-yukuai1@huaweicloud.com>
 <c96f2604-e1ef-c3ad-9d15-5e0efa5f222b@redhat.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <254fc651-aa75-074d-f567-49bafc437e9c@huaweicloud.com>
Date:   Wed, 14 Jun 2023 09:15:06 +0800
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Hi,

在 2023/06/13 22:43, Xiao Ni 写道:
> 
> 在 2023/5/29 下午9:20, Yu Kuai 写道:
>> From: Yu Kuai <yukuai3@huawei.com>
>>
>> Currently, for idle and frozen, action_store will hold 'reconfig_mutex'
>> and call md_reap_sync_thread() to stop sync thread, however, this will
>> cause deadlock (explained in the next patch). In order to fix the
>> problem, following patch will release 'reconfig_mutex' and wait on
>> 'resync_wait', like md_set_readonly() and do_md_stop() does.
>>
>> Consider that action_store() will set/clear 'MD_RECOVERY_FROZEN'
>> unconditionally, which might cause unexpected problems, for example,
>> frozen just set 'MD_RECOVERY_FROZEN' and is still in progress, while
>> 'idle' clear 'MD_RECOVERY_FROZEN' and new sync thread is started, which
>> might starve in progress frozen. A mutex is added to synchronize idle
>> and frozen from action_store().
>>
>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
>> ---
>>   drivers/md/md.c | 5 +++++
>>   drivers/md/md.h | 3 +++
>>   2 files changed, 8 insertions(+)
>>
>> diff --git a/drivers/md/md.c b/drivers/md/md.c
>> index 23e8e7eae062..63a993b52cd7 100644
>> --- a/drivers/md/md.c
>> +++ b/drivers/md/md.c
>> @@ -644,6 +644,7 @@ void mddev_init(struct mddev *mddev)
>>       mutex_init(&mddev->open_mutex);
>>       mutex_init(&mddev->reconfig_mutex);
>>       mutex_init(&mddev->delete_mutex);
>> +    mutex_init(&mddev->sync_mutex);
>>       mutex_init(&mddev->bitmap_info.mutex);
>>       INIT_LIST_HEAD(&mddev->disks);
>>       INIT_LIST_HEAD(&mddev->all_mddevs);
>> @@ -4785,14 +4786,18 @@ static void stop_sync_thread(struct mddev *mddev)
>>   static void idle_sync_thread(struct mddev *mddev)
>>   {
>> +    mutex_lock(&mddev->sync_mutex);
>>       clear_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
>>       stop_sync_thread(mddev);
>> +    mutex_unlock(&mddev->sync_mutex);
>>   }
>>   static void frozen_sync_thread(struct mddev *mddev)
>>   {
>> +    mutex_init(&mddev->delete_mutex);
> 
> 
> typo error? It should be mutex_lock(&mddev->sync_mutex); ?
> 

Yes, and thanks for spotting this, this looks like I did this while
rebasing.

Thanks,
Kuai
> Regards
> 
> Xiao
> 
>>       set_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
>>       stop_sync_thread(mddev);
>> +    mutex_unlock(&mddev->sync_mutex);
>>   }
>>   static ssize_t
>> diff --git a/drivers/md/md.h b/drivers/md/md.h
>> index bfd2306bc750..2fa903de5bd0 100644
>> --- a/drivers/md/md.h
>> +++ b/drivers/md/md.h
>> @@ -537,6 +537,9 @@ struct mddev {
>>       /* Protect the deleting list */
>>       struct mutex            delete_mutex;
>> +    /* Used to synchronize idle and frozen for action_store() */
>> +    struct mutex            sync_mutex;
>> +
>>       bool    has_superblocks:1;
>>       bool    fail_last_dev:1;
>>       bool    serialize_policy:1;
> 
> .
>