[PATCH v4] virtio-scsi: Fix the race condition in virtscsi_handle_event

[Fam Zheng <famz@redhat.com>]

There is a race condition in virtscsi_handle_event, when many device
hotplug/unplug events flush in quickly.

The scsi_remove_device in virtscsi_handle_transport_reset may trigger
the BUG_ON in scsi_target_reap, because the state is altered behind it,
probably by scsi_scan_host of another event. I'm able to reproduce it by
repeatedly plugging and unplugging a scsi disk with the same lun number.

To fix this, a single thread workqueue (local to the module) is added,
which makes the scan work serialized. With this change, the panic goes
away.

Signed-off-by: Fam Zheng <famz@redhat.com>

---

v4: Addressing MST's comments:
    Use ordered workqueue, with WQ_FREEZABLE and WQ_MEM_RECLAIM flags.
    Coding style fixes.

v3: Fix spacing and destroy order. (MST)
---
 drivers/scsi/virtio_scsi.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
index c52bb5d..0db63b5 100644
--- a/drivers/scsi/virtio_scsi.c
+++ b/drivers/scsi/virtio_scsi.c
@@ -120,6 +120,7 @@ struct virtio_scsi {
 
 static struct kmem_cache *virtscsi_cmd_cache;
 static mempool_t *virtscsi_cmd_pool;
+static struct workqueue_struct *virtscsi_scan_wq;
 
 static inline struct Scsi_Host *virtio_scsi_host(struct virtio_device *vdev)
 {
@@ -404,7 +405,7 @@ static void virtscsi_complete_event(struct virtio_scsi *vscsi, void *buf)
 	struct virtio_scsi_event_node *event_node = buf;
 
 	if (!vscsi->stop_events)
-		queue_work(system_freezable_wq, &event_node->work);
+		queue_work(virtscsi_scan_wq, &event_node->work);
 }
 
 static void virtscsi_event_done(struct virtqueue *vq)
@@ -1119,6 +1120,14 @@ static int __init init(void)
 		pr_err("mempool_create() for virtscsi_cmd_pool failed\n");
 		goto error;
 	}
+
+	virtscsi_scan_wq =
+		alloc_ordered_workqueue("virtscsi-scan", WQ_FREEZABLE | WQ_MEM_RECLAIM);
+	if (!virtscsi_scan_wq) {
+		pr_err("create_singlethread_workqueue() for virtscsi_scan_wq failed\n");
+		goto error;
+	}
+
 	ret = register_virtio_driver(&virtio_scsi_driver);
 	if (ret < 0)
 		goto error;
@@ -1126,6 +1135,8 @@ static int __init init(void)
 	return 0;
 
 error:
+	if (virtscsi_scan_wq)
+		destroy_workqueue(virtscsi_scan_wq);
 	if (virtscsi_cmd_pool) {
 		mempool_destroy(virtscsi_cmd_pool);
 		virtscsi_cmd_pool = NULL;
@@ -1140,6 +1151,7 @@ error:
 static void __exit fini(void)
 {
 	unregister_virtio_driver(&virtio_scsi_driver);
+	destroy_workqueue(virtscsi_scan_wq);
 	mempool_destroy(virtscsi_cmd_pool);
 	kmem_cache_destroy(virtscsi_cmd_cache);
 }
-- 
1.9.3