linux-kernel - Re: [PATCH] NVMe: Avoid interrupt disable during queue init.

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Date:	Fri, 22 May 2015 17:47:55 +0000 (UTC)
From:	Keith Busch <keith.busch@...el.com>
To:	Parav Pandit <parav.pandit@...gotech.com>
cc:	Keith Busch <keith.busch@...el.com>,
	linux-nvme@...ts.infradead.org,
	Matthew Wilcox <willy@...ux.intel.com>,
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] NVMe: Avoid interrupt disable during queue init.

On Fri, 22 May 2015, Parav Pandit wrote:
> I agree to it that nvmeq won't be null after mb(); That alone is not sufficient.
>
> What I have proposed in previous email is,
>
> Converting,
>
> struct nvme_queue *nvmeq = dev->queues[i];
> if (!nvmeq)
>    continue;
> spin_lock_irq(nvmeq->q_lock);
>
> to replace with,
>
> struct nvme_queue *nvmeq = rcu_dereference(dev->queues[i]);
> if (!nvmeq)
>    continue;
> spin_lock_irq(nvmeq->q_lock);
>
> This will prevent fetching content of q_lock before checking for NULL
> condition. Classic usage or RCU.

What the heck are you talking about? The value of dev->queue_count won't
even let the thread iterate an nvmeq before q_lock is initialized.

We used to rcu protect queue access, but that was to make nvme's
make_request_fn safe to surprise removal, not for the polling thread.
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