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Subject: Re: [PATCH -next 2/3] md/raid10: convert resync_lock to use seqlock
To:     Guoqing Jiang <guoqing.jiang@linux.dev>,
        Yu Kuai <yukuai1@huaweicloud.com>, song@kernel.org
Cc:     linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org,
        yi.zhang@huawei.com, "yukuai (C)" <yukuai3@huawei.com>
References: <20220829131502.165356-1-yukuai1@huaweicloud.com>
 <20220829131502.165356-3-yukuai1@huaweicloud.com>
 <3d8859bc-80d6-08b0-fd40-8874df4d3419@linux.dev>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <1891ec2c-0ccc-681e-31de-fdd28eebce82@huaweicloud.com>
Date:   Fri, 2 Sep 2022 18:02:05 +0800
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Hi,

在 2022/09/02 17:42, Guoqing Jiang 写道:
> Hi,
> 
> On 8/29/22 9:15 PM, Yu Kuai wrote:
>> +static bool wait_barrier_nolock(struct r10conf *conf)
>> +{
>> +    unsigned int seq = raw_read_seqcount(&conf->resync_lock.seqcount);
>> +
>> +    if (seq & 1)
>> +        return false;
>> +
>> +    if (READ_ONCE(conf->barrier))
>> +        return false;
>> +
>> +    atomic_inc(&conf->nr_pending);
>> +    if (!read_seqcount_retry(&conf->resync_lock.seqcount, seq))
> 
> I think 'seq' is usually get from read_seqcount_begin.

read_seqcount_begin will loop untill "req & 1" failed, I'm afraid this
will cause high cpu usage in come cases.

What I try to do here is just try once, and fall back to hold lock and
wait if failed.

What do you think?

Thanks,
Kuai
> 
>> +        return true;
>> +
>> +    atomic_dec(&conf->nr_pending);
>> +    return false;
>> +
> 
> Thanks,
> Guoqing
> .
>