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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>
References: <20210127103033.15318-1-dwagner@suse.de>
 <db9baae0-547c-7ff4-8b2c-0b95f14be67c@huawei.com>
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 <20210128094004.erwnszjqcxlsi2kd@beryllium.lan>
 <ebb1d098-3ded-e592-4419-e905aabe824f@huawei.com>
 <675d3cf7-1ae8-adc5-b6d0-359fe10f6b23@grimberg.me>
 <59cd053e-46cb-0235-141f-4ce919c93f48@huawei.com>
 <65392653-6b03-9195-f686-5fe4b3290bd2@suse.de>
 <81b22bbf-4dd3-6161-e63a-9699690a4e4f@huawei.com>
 <715dd943-0587-be08-2840-e0948cf0bc62@suse.de>
 <eb131d8f-f009-42e7-105d-58b84060f0dd@huawei.com>
 <ac019690-7f02-d28c-ed58-bfc8c1d48879@suse.de>
 <6ceff3cb-c9e9-7e74-92f0-dd745987c943@huawei.com>
From:   Hannes Reinecke <hare@suse.de>
Message-ID: <114751ac-1f7d-ce5e-12c5-7d6303bdb999@suse.de>
Date:   Mon, 1 Feb 2021 08:29:57 +0100
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On 2/1/21 3:16 AM, Chao Leng wrote:
> 
> 
> On 2021/1/29 17:20, Hannes Reinecke wrote:
>> On 1/29/21 9:46 AM, Chao Leng wrote:
>>>
>>>
>>> On 2021/1/29 16:33, Hannes Reinecke wrote:
>>>> On 1/29/21 8:45 AM, Chao Leng wrote:
>>>>>
>>>>>
>>>>> On 2021/1/29 15:06, Hannes Reinecke wrote:
>>>>>> 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.
>>>>> If there is just one path(the "old") and the "old" is deleted,
>>>>> nvme_next_ns() will return NULL.
>>>>> The list like this:
>>>>> head->next = head;
>>>>> old->next = head;
>>>>> If there is two or more path and the "old" is deleted,
>>>>> "for" will be infinite loop. because nvme_next_ns() will return
>>>>> the path which in the list except the "old", check condition will
>>>>> be true for ever.
>>>>
>>>> But that will be caught by the statement above:
>>>>
>>>> if (list_is_singular(&head->list))
>>>>
>>>> no?
>>> Two path just a sample example.
>>> If there is just two path, will enter it, may cause no path but there is
>>> actually one path. It is falsely assumed that the "old" must be not 
>>> deleted.
>>> If there is more than two path, will cause infinite loop.
>> So you mean we'll need something like this?
>>
>> diff --git a/drivers/nvme/host/multipath.c 
>> b/drivers/nvme/host/multipath.c
>> index 71696819c228..8ffccaf9c19a 100644
>> --- a/drivers/nvme/host/multipath.c
>> +++ b/drivers/nvme/host/multipath.c
>> @@ -202,10 +202,12 @@ static struct nvme_ns *__nvme_find_path(struct 
>> nvme_ns_head *head, int node)
>>   static struct nvme_ns *nvme_next_ns(struct nvme_ns_head *head,
>>                  struct nvme_ns *ns)
>>   {
>> -       ns = list_next_or_null_rcu(&head->list, &ns->siblings, struct 
>> nvme_ns,
>> -                       siblings);
>> -       if (ns)
>> -               return ns;
>> +       if (ns) {
>> +               ns = list_next_or_null_rcu(&head->list, &ns->siblings,
>> +                                          struct nvme_ns, siblings);
>> +               if (ns)
>> +                       return ns;
>> +       }
> No, in the scenario, ns should not be NULL.

Why not? 'ns == NULL' is precisely the corner-case this is trying to fix...

> May be we can do like this:
> 
> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> index 282b7a4ea9a9..b895011a2cbd 100644
> --- a/drivers/nvme/host/multipath.c
> +++ b/drivers/nvme/host/multipath.c
> @@ -199,30 +199,24 @@ static struct nvme_ns *__nvme_find_path(struct 
> nvme_ns_head *head, int node)
>          return found;
>   }
> 
> -static struct nvme_ns *nvme_next_ns(struct nvme_ns_head *head,
> -               struct nvme_ns *ns)
> -{
> -       ns = list_next_or_null_rcu(&head->list, &ns->siblings, struct 
> nvme_ns,
> -                       siblings);
> -       if (ns)
> -               return ns;
> -       return list_first_or_null_rcu(&head->list, struct nvme_ns, 
> siblings);
> -}
> +#define nvme_next_ns_condition(head, current, condition) \
> +({ \
> +       struct nvme_ns *__ptr = list_next_or_null_rcu(&(head)->list, \
> +               &(current)->siblings, struct nvme_ns, siblings); \
> +       __ptr ? __ptr : (condition) ? (condition) = false, \
> +               list_first_or_null_rcu(&(head)->list, struct nvme_ns, \
> +                       siblings) : NULL; \
> +})
> 
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.

Cheers,

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