[PATCH] nvme: fix deadlock between reset and scan
yaoma
yaoma at linux.alibaba.com
Tue Nov 28 19:28:40 PST 2023
On 2023/11/28 18:13, Sagi Grimberg wrote:
>
>
> On 11/28/23 08:22, yaoma wrote:
>> Hi Keith Busch
>>
>> Thanks for your reply.
>>
>> The idea to avoid such a deadlock between nvme_reset and nvme_scan is
>> to ensure that no namespace can be added to ctrl->namespaces after
>> nvme_start_freeze has already been called. We can achieve this goal by
>> assessing the ctrl->state after we have already acquired the
>> ctrl->namespaces_rwsem lock, to decide whether to add the namespace to
>> the list or not.
>> 1. After we determine that ctrl->state is LIVE, it may be immediately
>> changed to another state. However, since we have already acquired the
>> lock, other tasks cannot access ctrl->namespace, so we can still
>> safely add the namespace to the list. After acquiring the lock,
>> nvme_start_freeze will freeze all ns->q in the list, including any
>> newly added namespaces.
>> 2. Before the completion of nvme_reset, ctrl->state will not be
>> changed to LIVE, so we will not add any more namespaces to the list.
>> All ns->q in the list is frozen, so nvme_wait_freeze can exit normally.
>
> I agree with the analysis, there is a hole between start_freeze and
> freeze_wait that a scan may add a ns to the ctrl ns list.
>
I think your proposal is nice, and I will test it.
> However the fix should be to mark the ctrl with say NVME_CTRL_FROZEN
> flag set in nvme_freeze_start and cleared in nvme_unfreeze (similar
> to what we did with quiesce). Then the scan can check it before adding
> the new namespace (under the namespaces_rwsem).
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