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

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Message-ID: <20180515075221.4ppaxdvbosy36ltk@linux-x5ow.site>
Date:   Tue, 15 May 2018 09:52:21 +0200
From:   Johannes Thumshirn <jthumshirn@...e.de>
To:     Christoph Hellwig <hch@....de>
Cc:     Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        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 09:50:55AM +0200, Christoph Hellwig wrote:
> 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?

__blk_mq_run_hw_queue() calls blk_mq_sched_dispatch_requests() with
the hctx_lock() a.k.a rcu_read_lock() held.

nvmf_connect_admin_queue() does I/O which causes the above to be called.

> 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.

Hmm I'll be looking into it.

-- 
Johannes Thumshirn                                          Storage
jthumshirn@...e.de                                +49 911 74053 689
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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