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Message-ID: <6ceff3cb-c9e9-7e74-92f0-dd745987c943@huawei.com>
Date: Mon, 1 Feb 2021 10:16:35 +0800
From: Chao Leng <lengchao@...wei.com>
To: Hannes Reinecke <hare@...e.de>, 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 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.
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; \
+})
static struct nvme_ns *nvme_round_robin_path(struct nvme_ns_head *head,
int node, struct nvme_ns *old)
{
struct nvme_ns *ns, *found = NULL;
+ bool first_half = true;
- if (list_is_singular(&head->list)) {
- if (nvme_path_is_disabled(old))
- return NULL;
- return old;
- }
-
- for (ns = nvme_next_ns(head, old);
+ for (ns = nvme_next_ns_condition(head, old, first_half);
ns && ns != old;
- ns = nvme_next_ns(head, ns)) {
+ ns = nvme_next_ns_condition(head, ns, first_half)) {
if (nvme_path_is_disabled(ns))
continue;
> return list_first_or_null_rcu(&head->list, struct nvme_ns, siblings);
> }
>
> Cheers,
>
> Hannes
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