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

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Date:	Fri, 22 May 2015 22:18:43 +0530
From:	Parav Pandit <parav.pandit@...gotech.com>
To:	Keith Busch <keith.busch@...el.com>
Cc:	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, May 22, 2015 at 9:53 PM, Keith Busch <keith.busch@...el.com> wrote:
> On Fri, 22 May 2015, Parav Pandit wrote:
>>
>> During normal positive path probe,
>> (a) device is added to dev_list in nvme_dev_start()
>> (b) nvme_kthread got created, which will eventually refers to
>> dev->queues[qid] to check for NULL.
>> (c) dev_start() worker thread has started probing device and creating
>> the queue using nvme_alloc_queue
>> This is is assigning the dev->queue[qid] new pointer.
>> If this is done out of order, nvme_kthread will pickup uninitialized
>> q_lock, cq_phase, q_db.
>
>
> A memory barrier before incrementing the dev->queue_count (and assigning
> the pointer in the array before that) should address this concern.
>

Sure. mb() will solve the publisher side problem. RCU is wrapper around mb().
However mb() doesn't solve the issue of q_lock variable getting
fetched before if (!nvmeq) condition being executed, by value
compilation optimizations in nvme_kthread().
So I was inclined towards more preferred method of rcu.


>> Other thoughts to not create nvme_kthread until all the queues are active.
>
>
> No good, we want to poll during queue creation to detect controller
> errors and broken interrupts.
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