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

[Sagi Grimberg]

>> 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).

nvme_ns_remove fences continue of deletion of the ns by synchronizing
the srcu such that for sure the current_path clearance is visible.