Re: [PATCH v2] nvme-multipath: Early exit if no path is available

[Chao Leng <lengchao@huawei.com>]

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.
For example, there is two path: one is the "old", another is "ns1".
The list like this:
head->next = ns1;
ns1->next = head;
old->next = ns1;
nvme_next_ns(head, old) will return ns1;
Then nvme_next_ns(head, ns1) will return ns1;
And then infinite loop.
> 
> Cheers,
> 
> Hannes

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