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Date:   Wed, 29 Nov 2023 11:28:40 +0800
From:   yaoma <yaoma@...ux.alibaba.com>
To:     Sagi Grimberg <sagi@...mberg.me>, Keith Busch <kbusch@...nel.org>
Cc:     axboe@...nel.dk, hch@....de, linux-nvme@...ts.infradead.org,
        linux-kernel@...r.kernel.org, kanie@...ux.alibaba.com
Subject: Re: [PATCH] nvme: fix deadlock between reset and scan



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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