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Subject: Re: [PATCH v2] nvme-multipath: Early exit if no path is available
To:     Chao Leng <lengchao@huawei.com>, Sagi Grimberg <sagi@grimberg.me>,
        Daniel Wagner <dwagner@suse.de>
Cc:     linux-nvme@lists.infradead.org, linux-kernel@vger.kernel.org,
        Jens Axboe <axboe@fb.com>, Keith Busch <kbusch@kernel.org>,
        Christoph Hellwig <hch@lst.de>
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From:   Hannes Reinecke <hare@suse.de>
Message-ID: <65392653-6b03-9195-f686-5fe4b3290bd2@suse.de>
Date:   Fri, 29 Jan 2021 08:06:57 +0100
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On 1/29/21 4:07 AM, Chao Leng wrote:
> 
> 
> On 2021/1/29 9:42, Sagi Grimberg wrote:
>>
>>>> You can't see exactly where it dies but I followed the assembly to
>>>> nvme_round_robin_path(). Maybe it's not the initial nvme_next_ns(head,
>>>> old) which returns NULL but nvme_next_ns() is returning NULL eventually
>>>> (list_next_or_null_rcu()).
>>> So there is other bug cause nvme_next_ns abormal.
>>> I review the code about head->list and head->current_path, I find 2 bugs
>>> may cause the bug:
>>> First, I already send the patch. see:
>>> https://lore.kernel.org/linux-nvme/20210128033351.22116-1-lengchao@huawei.com/ 
>>>
>>> Second, in nvme_ns_remove, list_del_rcu is before
>>> nvme_mpath_clear_current_path. This may cause "old" is deleted from the
>>> "head", but still use "old". I'm not sure there's any other
>>> consideration here, I will check it and try to fix it.
>>
>> The reason why we first remove from head->list and only then clear
>> current_path is because the other way around there is no way
>> to guarantee that that the ns won't be assigned as current_path
>> again (because it is in head->list).
> ok, I see.
>>
>> nvme_ns_remove fences continue of deletion of the ns by synchronizing
>> the srcu such that for sure the current_path clearance is visible.
> The list will be like this:
> head->next = ns1;
> ns1->next = head;
> old->next = ns1;

Where does 'old' pointing to?

> This may cause infinite loop in nvme_round_robin_path.
> for (ns = nvme_next_ns(head, old);
>      ns != old;
>      ns = nvme_next_ns(head, ns))
> The ns will always be ns1, and then infinite loop.

No. nvme_next_ns() will return NULL.

Cheers,

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