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Subject: Re: [PATCH v2] nvme-multipath: Early exit if no path is available
To:     Chao Leng <lengchao@huawei.com>, Daniel Wagner <dwagner@suse.de>
Cc:     linux-nvme@lists.infradead.org, Sagi Grimberg <sagi@grimberg.me>,
        linux-kernel@vger.kernel.org, Jens Axboe <axboe@fb.com>,
        Keith Busch <kbusch@kernel.org>, Christoph Hellwig <hch@lst.de>
References: <20210127103033.15318-1-dwagner@suse.de>
 <db9baae0-547c-7ff4-8b2c-0b95f14be67c@huawei.com>
 <20210128075837.u5u56t23fq5gu6ou@beryllium.lan>
 <69575290-200e-b4a1-4269-c71e4c2cc37b@huawei.com>
From:   Hannes Reinecke <hare@suse.de>
Message-ID: <8704038d-0b71-5c9c-2d2b-a9ff2166dbe4@suse.de>
Date:   Thu, 28 Jan 2021 10:23:42 +0100
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On 1/28/21 10:18 AM, Chao Leng wrote:
> 
> 
> On 2021/1/28 15:58, Daniel Wagner wrote:
>> On Thu, Jan 28, 2021 at 09:31:30AM +0800, Chao Leng wrote:
>>>> --- a/drivers/nvme/host/multipath.c
>>>> +++ b/drivers/nvme/host/multipath.c
>>>> @@ -221,7 +221,7 @@ static struct nvme_ns 
>>>> *nvme_round_robin_path(struct nvme_ns_head *head,
>>>>        }
>>>>        for (ns = nvme_next_ns(head, old);
>>>> -         ns != old;
>>>> +         ns && ns != old;
>>> nvme_round_robin_path just be called when !"old".
>>> nvme_next_ns should not return NULL when !"old".
>>> It seems unnecessary to add checking "ns".
>>
>> The problem is when we enter nvme_round_robin_path() and there is no
>> path available. In this case the initialization ns = nvme_next_ns(head,
>> old) could return a NULL pointer."old" should not be NULL, so there is 
>> at least one path that is "old".
> It is impossible to return NULL for nvme_next_ns(head, old).

No. list_next_or_null_rcu()/list_first_or_null_rcu() will return NULL 
when then end of the list is reached.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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