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

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Date:   Mon, 1 Feb 2021 11:45:26 +0100
From:   Hannes Reinecke <hare@...e.de>
To:     Chao Leng <lengchao@...wei.com>, Sagi Grimberg <sagi@...mberg.me>,
        Daniel Wagner <dwagner@...e.de>
Cc:     linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...com>, Keith Busch <kbusch@...nel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2] nvme-multipath: Early exit if no path is available

On 2/1/21 10:40 AM, Chao Leng wrote:
> 
> 
> 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.

Ah. So this should be better:

@@ -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 && !test_bit(NVME_NS_REMOVING, &ns->flags)) {
+               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);
  }

The 'NVME_NS_REMOVING' bit is set before list_del_rcu() is called, so it 
should guard against the issue you mentioned.

Cheers,

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