linux-kernel - Re: [PATCH] nvme: fix deadlock between reset and scan

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Date:   Tue, 28 Nov 2023 12:13:59 +0200
From:   Sagi Grimberg <sagi@...mberg.me>
To:     yaoma <yaoma@...ux.alibaba.com>, 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 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.

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).