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

[Parav Pandit <Parav.pandit@avagotech.com>]

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;
 
@@ -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);
-- 
1.7.1