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Subject: Re: [PATCH v2] nvme-multipath: Early exit if no path is available
To:     Hannes Reinecke <hare@suse.de>, 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:   Chao Leng <lengchao@huawei.com>
Message-ID: <bd37abd5-759d-efe2-fdcd-8b004a41c75a@huawei.com>
Date:   Mon, 1 Feb 2021 17:40:40 +0800
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On 2021/2/1 16:57, Hannes Reinecke wrote:
> On 2/1/21 9:47 AM, Chao Leng wrote:
>>
>>
>> On 2021/2/1 15:29, Hannes Reinecke wrote:[ .. ]
>>> Urgh. Please, no. That is well impossible to debug.
>>> Can you please open-code it to demonstrate where the difference to the current (and my fixed) versions is?
>>> I'm still not clear where the problem is once we applied both patches.
>> For example assume the list has three path, and all path is not NVME_ANA_OPTIMIZED:
>> head->next = ns1;
>> ns1->next = ns2;
>> ns2->next = head;
>> old->next = ns2;
>>
> And this is where I have issues with.
> Where does 'old' come from?
> Clearly it was part of the list at one point; so what happened to it?
I explained this earlier.
In nvme_ns_remove, there is a hole between list_del_rcu and
nvme_mpath_clear_current_path. If head->current_path is the "old", and
the "old" is removing. The "old" is already removed from the list by
list_del_rcu, but head->current_path is not clear to NULL by
nvme_mpath_clear_current_path.
Find path is race with nvme_ns_remove, use the "old" pass to
nvme_round_robin_path to find path.
> 
> Cheers,
> 
> Hannes