[PATCH v2] virtio-blk: handle block_device_operations callbacks after hot unplug

[Stefan Hajnoczi <stefanha@redhat.com>]

A userspace process holding a file descriptor to a virtio_blk device can
still invoke block_device_operations after hot unplug.  For example, a
program that has /dev/vdb open can call ioctl(HDIO_GETGEO) after hot
unplug to invoke virtblk_getgeo().

Introduce a reference count in struct virtio_blk so that its lifetime
covers both virtio_driver probe/remove and block_device_operations
open/release users.  This ensures that block_device_operations functions
like virtblk_getgeo() can safely access struct virtio_blk.

Add remove_mutex to prevent block_device_operations functions from
accessing vblk->vdev during virtblk_remove() and let the safely check
for !vblk->vdev after virtblk_remove() returns.

Switching to a reference count also solves the vd_index_ida leak where
vda, vdb, vdc, etc indices were lost when the device was hot unplugged
while the block device was still open.

Reported-by: Lance Digby <ldigby@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
If someone has a simpler solution please let me know.  I looked at
various approaches including reusing device_lock(&vblk->vdev.dev) but
they were more complex and extending the lifetime of virtio_device after
remove() has been called seems questionable.
---
 drivers/block/virtio_blk.c | 85 ++++++++++++++++++++++++++++++++++----
 1 file changed, 77 insertions(+), 8 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 93468b7c6701..3dd53b445cc1 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -44,6 +44,13 @@ struct virtio_blk {
 	/* Process context for config space updates */
 	struct work_struct config_work;
 
+	/*
+	 * Tracks references from block_device_operations open/release and
+	 * virtio_driver probe/remove so this object can be freed once no
+	 * longer in use.
+	 */
+	refcount_t refs;
+
 	/* What host tells us, plus 2 for header & tailer. */
 	unsigned int sg_elems;
 
@@ -53,6 +60,9 @@ struct virtio_blk {
 	/* num of vqs */
 	int num_vqs;
 	struct virtio_blk_vq *vqs;
+
+	/* Provides mutual exclusion with virtblk_remove(). */
+	struct mutex remove_mutex;
 };
 
 struct virtblk_req {
@@ -295,10 +305,54 @@ static int virtblk_get_id(struct gendisk *disk, char *id_str)
 	return err;
 }
 
+static void virtblk_get(struct virtio_blk *vblk)
+{
+	refcount_inc(&vblk->refs);
+}
+
+static void virtblk_put(struct virtio_blk *vblk)
+{
+	if (refcount_dec_and_test(&vblk->refs)) {
+		ida_simple_remove(&vd_index_ida, vblk->index);
+		mutex_destroy(&vblk->remove_mutex);
+		kfree(vblk);
+	}
+}
+
+static int virtblk_open(struct block_device *bd, fmode_t mode)
+{
+	struct virtio_blk *vblk = bd->bd_disk->private_data;
+	int ret = -ENXIO;
+
+	mutex_lock(&vblk->remove_mutex);
+
+	if (vblk->vdev) {
+		virtblk_get(vblk);
+		ret = 0;
+	}
+
+	mutex_unlock(&vblk->remove_mutex);
+	return ret;
+}
+
+static void virtblk_release(struct gendisk *disk, fmode_t mode)
+{
+	struct virtio_blk *vblk = disk->private_data;
+
+	virtblk_put(vblk);
+}
+
 /* We provide getgeo only to please some old bootloader/partitioning tools */
 static int virtblk_getgeo(struct block_device *bd, struct hd_geometry *geo)
 {
 	struct virtio_blk *vblk = bd->bd_disk->private_data;
+	int ret = -ENXIO;
+
+	mutex_lock(&vblk->remove_mutex);
+
+	if (!vblk->vdev) {
+		goto out;
+	}
 
 	/* see if the host passed in geometry config */
 	if (virtio_has_feature(vblk->vdev, VIRTIO_BLK_F_GEOMETRY)) {
@@ -314,11 +368,17 @@ static int virtblk_getgeo(struct block_device *bd, struct hd_geometry *geo)
 		geo->sectors = 1 << 5;
 		geo->cylinders = get_capacity(bd->bd_disk) >> 11;
 	}
-	return 0;
+
+	ret = 0;
+out:
+	mutex_unlock(&vblk->remove_mutex);
+	return ret;
 }
 
 static const struct block_device_operations virtblk_fops = {
 	.owner  = THIS_MODULE,
+	.open = virtblk_open,
+	.release = virtblk_release,
 	.getgeo = virtblk_getgeo,
 };
 
@@ -655,6 +715,10 @@ static int virtblk_probe(struct virtio_device *vdev)
 		goto out_free_index;
 	}
 
+	/* This reference is dropped in virtblk_remove(). */
+	refcount_set(&vblk->refs, 1);
+	mutex_init(&vblk->remove_mutex);
+
 	vblk->vdev = vdev;
 	vblk->sg_elems = sg_elems;
 
@@ -820,8 +884,12 @@ static int virtblk_probe(struct virtio_device *vdev)
 static void virtblk_remove(struct virtio_device *vdev)
 {
 	struct virtio_blk *vblk = vdev->priv;
-	int index = vblk->index;
-	int refc;
+
+	/*
+	 * Virtqueue processing is stopped safely here but mutual exclusion is
+	 * needed for block_device_operations.
+	 */
+	mutex_lock(&vblk->remove_mutex);
 
 	/* Make sure no work handler is accessing the device. */
 	flush_work(&vblk->config_work);
@@ -834,15 +902,16 @@ static void virtblk_remove(struct virtio_device *vdev)
 	/* Stop all the virtqueues. */
 	vdev->config->reset(vdev);
 
-	refc = kref_read(&disk_to_dev(vblk->disk)->kobj.kref);
+	/* Virtqueue are stopped, nothing can use vblk->vdev anymore. */
+	vblk->vdev = NULL;
+
 	put_disk(vblk->disk);
 	vdev->config->del_vqs(vdev);
 	kfree(vblk->vqs);
-	kfree(vblk);
 
-	/* Only free device id if we don't have any users */
-	if (refc == 1)
-		ida_simple_remove(&vd_index_ida, index);
+	mutex_unlock(&vblk->remove_mutex);
+
+	virtblk_put(vblk);
 }
 
 #ifdef CONFIG_PM_SLEEP
-- 
2.25.3