linux-kernel - Re: [PATCH 3/5] nvme: call nvmf_create_ctrl before checking for duplicate assignment

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Message-ID: <20180515075055.GB23659@lst.de>
Date:   Tue, 15 May 2018 09:50:55 +0200
From:   Christoph Hellwig <hch@....de>
To:     Johannes Thumshirn <jthumshirn@...e.de>
Cc:     Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Christoph Hellwig <hch@....de>,
        Linux NVMe Mailinglist <linux-nvme@...ts.infradead.org>,
        Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>,
        Hannes Reinecke <hare@...e.de>
Subject: Re: [PATCH 3/5] nvme: call nvmf_create_ctrl before checking for
        duplicate assignment

On Tue, May 15, 2018 at 09:40:41AM +0200, Johannes Thumshirn wrote:
> In nvmf_dev_write we did check if the /dev/nvme-fabrics device node's
> private data is already set and then create a controller data
> structure afterwards. The private data is protected by the
> nvmf_dev_mutex, but there is no need to hold it while calling
> nvmf_create_ctrl().
> 
> This also reduces the number of lockdep complaints in the 'nvme
> connect' with fcloop scenario.

Something looks seriously fishy in fcloop.  How do we and up in
a RCU critical section here?

In general it seems like fc loop needs to offload any I/O to a workqueue
just like nvme-loop does, but even then I can't see how that is going to
cause an issue in this area.