[PATCH 1/1] nvme-pci: avoid race between nvme_reap_pending_cqes() and nvme_poll()

[Dongli Zhang]

There may be a race between nvme_reap_pending_cqes() and nvme_poll(), e.g.,
when doing live reset while polling the nvme device.

      CPU X                        CPU Y
                               nvme_poll()
nvme_dev_disable()
-> nvme_stop_queues()
-> nvme_suspend_io_queues()
-> nvme_suspend_queue()
                               -> spin_lock(&nvmeq->cq_poll_lock);
-> nvme_reap_pending_cqes()
   -> nvme_process_cq()        -> nvme_process_cq()

In the above scenario, the nvme_process_cq() for the same queue may be
running on both CPU X and CPU Y concurrently.

It is much more easier to reproduce the issue when CONFIG_PREEMPT is
enabled in kernel. When CONFIG_PREEMPT is disabled, it would take longer
time for nvme_stop_queues()-->blk_mq_quiesce_queue() to wait for grace
period.

This patch protects nvme_process_cq() with nvmeq->cq_poll_lock in
nvme_reap_pending_cqes().

Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
---
 drivers/nvme/host/pci.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
index 3726dc780d15..cc46e250fcac 100644
--- a/drivers/nvme/host/pci.c
+++ b/drivers/nvme/host/pci.c
@@ -1382,16 +1382,19 @@ static void nvme_disable_admin_queue(struct nvme_dev *dev, bool shutdown)
 
 /*
  * Called only on a device that has been disabled and after all other threads
- * that can check this device's completion queues have synced. This is the
- * last chance for the driver to see a natural completion before
- * nvme_cancel_request() terminates all incomplete requests.
+ * that can check this device's completion queues have synced, except
+ * nvme_poll(). This is the last chance for the driver to see a natural
+ * completion before nvme_cancel_request() terminates all incomplete requests.
  */
 static void nvme_reap_pending_cqes(struct nvme_dev *dev)
 {
 	int i;
 
-	for (i = dev->ctrl.queue_count - 1; i > 0; i--)
+	for (i = dev->ctrl.queue_count - 1; i > 0; i--) {
+		spin_lock(&dev->queues[i].cq_poll_lock);
 		nvme_process_cq(&dev->queues[i]);
+		spin_unlock(&dev->queues[i].cq_poll_lock);
+	}
 }
 
 static int nvme_cmb_qdepth(struct nvme_dev *dev, int nr_io_queues,
-- 
2.17.1