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Subject: Re: [dm-devel] [PATCH -next v2 4/6] md: refactor
 idle/frozen_sync_thread() to fix deadlock
To:     Yu Kuai <yukuai1@huaweicloud.com>, Xiao Ni <xni@redhat.com>,
        guoqing.jiang@linux.dev, agk@redhat.com, snitzer@kernel.org,
        dm-devel@redhat.com, song@kernel.org
Cc:     linux-raid@vger.kernel.org, yangerkun@huawei.com,
        linux-kernel@vger.kernel.org, yi.zhang@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20230529132037.2124527-1-yukuai1@huaweicloud.com>
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 <05aa3b09-7bb9-a65a-6231-4707b4b078a0@redhat.com>
 <74b404c4-4fdb-6eb3-93f1-0e640793bba6@huaweicloud.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <6e738d9b-6e92-20b7-f9d9-e1cf71d26d73@huaweicloud.com>
Date:   Wed, 14 Jun 2023 10:04:33 +0800
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Hi,

在 2023/06/14 9:48, Yu Kuai 写道:


>>
>> In the patch, sync_seq is added in md_reap_sync_thread. In 
>> idle_sync_thread, if sync_seq isn't equal
>>
>> mddev->sync_seq, it should mean there is someone that stops the sync 
>> thread already, right? Why do
>>
>> you say 'new started sync thread' here?

If someone stops the sync thread, and new sync thread is not started,
then this sync_seq won't make a difference, above wait_event() will not
wait because !test_bit(MD_RECOVERY_RUNNING, &mddev->recovery) will pass.
So 'sync_seq' is only used when the old sync thread stops and new sync
thread starts, add 'sync_seq' will bypass this case.

Thanks,
Kuai