Messages in this thread | | | Subject | Re: [PATCH v2] nvme-multipath: Early exit if no path is available | From | Chao Leng <> | Date | Fri, 29 Jan 2021 11:36:08 +0800 |
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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.
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