Re: [PATCH] NVMe: Fixed race between nvme_thread & probe path.

[Jon Derrick <jonathan.derrick@intel.com>]

On Thu, Jun 18, 2015 at 04:13:50PM +0530, Parav Pandit wrote:
> Kernel thread nvme_thread and driver load process can be executing
> in parallel on different CPU. This leads to race condition whenever
> nvme_alloc_queue() instructions are executed out of order that can
> reflects incorrect value for nvme_thread.
> Memory barrier in nvme_alloc_queue() ensures that it maintains the
> order and and data dependency read barrier in reader thread ensures
> that cpu cache is synced.
> 
> Signed-off-by: Parav Pandit <Parav.pandit@avagotech.com>
> ---
>  drivers/block/nvme-core.c |   12 ++++++++++--
>  1 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/block/nvme-core.c b/drivers/block/nvme-core.c
> index 5961ed7..90fb0ce 100644
> --- a/drivers/block/nvme-core.c
> +++ b/drivers/block/nvme-core.c
> @@ -1403,8 +1403,10 @@ static struct nvme_queue *nvme_alloc_queue(struct nvme_dev *dev, int qid,
>  	nvmeq->q_db = &dev->dbs[qid * 2 * dev->db_stride];
>  	nvmeq->q_depth = depth;
>  	nvmeq->qid = qid;
> -	dev->queue_count++;
>  	dev->queues[qid] = nvmeq;
> +	/* update queues first before updating queue_count */
> +	smp_wmb();
> +	dev->queue_count++;
>  
>  	return nvmeq;
>  

This has been applied already as an explicit mb()

> @@ -2073,7 +2075,13 @@ static int nvme_kthread(void *data)
>  				continue;
>  			}
>  			for (i = 0; i < dev->queue_count; i++) {
> -				struct nvme_queue *nvmeq = dev->queues[i];
> +				struct nvme_queue *nvmeq;
> +
> +				/* make sure to read queue_count before
> +				 * traversing queues.
> +				 */
> +				smp_read_barrier_depends();
> +				nvmeq = dev->queues[i];
>  				if (!nvmeq)
>  					continue;
>  				spin_lock_irq(&nvmeq->q_lock);

I don't think this is necessary. If queue_count is incremented while in this loop, it will be picked up the next time the kthread runs