From: Chao Leng <lengchao@huawei.com>
To: 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>, Hannes Reinecke <hare@suse.de>,
Keith Busch <kbusch@kernel.org>, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v2] nvme-multipath: Early exit if no path is available
Date: Fri, 29 Jan 2021 11:36:08 +0800 [thread overview]
Message-ID: <a3c64c60-13f3-1b99-47bc-05ebfefed343@huawei.com> (raw)
In-Reply-To: <dbc39e56-b2bd-a82e-499a-5032c9716855@grimberg.me>
On 2021/1/29 11:30, 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;
>> 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.
>
> Who is being removed? I'm not following
The "old" is being removed path.
Daniel Wagner report crash like this:
head->next = head;
old->next = head;
So nvme_next_ns(head, old) will return NULL, and then crash.
Although check ns can avoid crash, but can not avoid infinite loop.
Similar reason, The list will be like this:
head->next = ns1;
ns1->next = head;
old->next = ns1;
ns1 is other path.
> .
next prev parent reply other threads:[~2021-01-29 3:37 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-27 10:30 [PATCH v2] nvme-multipath: Early exit if no path is available Daniel Wagner
2021-01-27 10:34 ` Hannes Reinecke
2021-01-27 16:49 ` Christoph Hellwig
2021-01-28 1:31 ` Chao Leng
2021-01-28 7:58 ` Daniel Wagner
2021-01-28 9:18 ` Chao Leng
2021-01-28 9:23 ` Hannes Reinecke
2021-01-29 1:18 ` Chao Leng
2021-01-28 9:40 ` Daniel Wagner
2021-01-29 1:23 ` Chao Leng
2021-01-29 1:42 ` Sagi Grimberg
2021-01-29 3:07 ` Chao Leng
2021-01-29 3:30 ` Sagi Grimberg
2021-01-29 3:36 ` Chao Leng [this message]
2021-01-29 7:06 ` Hannes Reinecke
[not found] ` <81b22bbf-4dd3-6161-e63a-9699690a4e4f@huawei.com>
[not found] ` <715dd943-0587-be08-2840-e0948cf0bc62@suse.de>
[not found] ` <eb131d8f-f009-42e7-105d-58b84060f0dd@huawei.com>
[not found] ` <ac019690-7f02-d28c-ed58-bfc8c1d48879@suse.de>
2021-02-01 2:16 ` Chao Leng
2021-02-01 7:29 ` Hannes Reinecke
2021-02-01 8:47 ` Chao Leng
2021-02-01 8:57 ` Hannes Reinecke
2021-02-01 9:40 ` Chao Leng
2021-02-01 10:45 ` Hannes Reinecke
2021-02-02 1:12 ` Chao Leng
2021-01-28 1:36 ` Chao Leng
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