[PATCH 1/2] virtio-blk: Fix hot-unplug race in remove method

[Asias He <asias@redhat.com>]

If we reset the virtio-blk device before the requests already dispatched
to the virtio-blk driver from the block layer are finised, we will stuck
in blk_cleanup_queue() and the remove will fail.

blk_cleanup_queue() calls blk_drain_queue() to drain all requests queued
before DEAD marking. However it will never success if the device is
already stopped. We'll have q->in_flight[] > 0, so the drain will not
finish.

How to reproduce the race:
1. hot-plug a virtio-blk device
2. keep reading/writing the device in guest
3. hot-unplug while the device is busy serving I/O

Signed-off-by: Asias He <asias@redhat.com>
---
 drivers/block/virtio_blk.c |   26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 72fe55d..72b818b 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -46,6 +46,9 @@ struct virtio_blk
 	/* Ida index - used to track minor number allocations. */
 	int index;
 
+	/* Number of pending requests dispatched to driver. */
+	int req_in_flight;
+
 	/* Scatterlist: can be too big for stack. */
 	struct scatterlist sg[/*sg_elems*/];
 };
@@ -95,6 +98,7 @@ static void blk_done(struct virtqueue *vq)
 		}
 
 		__blk_end_request_all(vbr->req, error);
+		vblk->req_in_flight--;
 		mempool_free(vbr, vblk->pool);
 	}
 	/* In case queue is stopped waiting for more buffers. */
@@ -190,6 +194,7 @@ static void do_virtblk_request(struct request_queue *q)
 
 	while ((req = blk_peek_request(q)) != NULL) {
 		BUG_ON(req->nr_phys_segments + 2 > vblk->sg_elems);
+		vblk->req_in_flight++;
 
 		/* If this request fails, stop queue and wait for something to
 		   finish to restart it. */
@@ -443,7 +448,7 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
 	if (err)
 		goto out_free_vblk;
 
-	vblk->pool = mempool_create_kmalloc_pool(1,sizeof(struct virtblk_req));
+	vblk->pool = mempool_create_kmalloc_pool(1, sizeof(struct virtblk_req));
 	if (!vblk->pool) {
 		err = -ENOMEM;
 		goto out_free_vq;
@@ -466,6 +471,7 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
 
 	virtblk_name_format("vd", index, vblk->disk->disk_name, DISK_NAME_LEN);
 
+	vblk->req_in_flight = 0;
 	vblk->disk->major = major;
 	vblk->disk->first_minor = index_to_minor(index);
 	vblk->disk->private_data = vblk;
@@ -576,22 +582,34 @@ static void __devexit virtblk_remove(struct virtio_device *vdev)
 {
 	struct virtio_blk *vblk = vdev->priv;
 	int index = vblk->index;
+	unsigned long flags;
+	int req_in_flight;
 
 	/* Prevent config work handler from accessing the device. */
 	mutex_lock(&vblk->config_lock);
 	vblk->config_enable = false;
 	mutex_unlock(&vblk->config_lock);
 
+	/* Abort all request on the queue. */
+	blk_abort_queue(vblk->disk->queue);
+	del_gendisk(vblk->disk);
+
 	/* Stop all the virtqueues. */
 	vdev->config->reset(vdev);
-
+	vdev->config->del_vqs(vdev);
 	flush_work(&vblk->config_work);
 
-	del_gendisk(vblk->disk);
+	/* Wait requests dispatched to device driver to finish. */
+	do {
+		spin_lock_irqsave(&vblk->lock, flags);
+		req_in_flight = vblk->req_in_flight;
+		spin_unlock_irqrestore(&vblk->lock, flags);
+	} while (req_in_flight != 0);
+
 	blk_cleanup_queue(vblk->disk->queue);
 	put_disk(vblk->disk);
+
 	mempool_destroy(vblk->pool);
-	vdev->config->del_vqs(vdev);
 	kfree(vblk);
 	ida_simple_remove(&vd_index_ida, index);
 }
-- 
1.7.10