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[PATCH 2/2] virtio-scsi: stop using aio_disable_external() during unplug

From: Stefan Hajnoczi
Subject: [PATCH 2/2] virtio-scsi: stop using aio_disable_external() during unplug
Date: Thu, 23 Mar 2023 14:56:07 -0400

This patch is part of an effort to remove the aio_disable_external()
API because it does not fit in a multi-queue block layer world where
many AioContexts may be submitting requests to the same disk.

The SCSI emulation code is already in good shape to stop using
aio_disable_external(). The API is only used by commit 9c5aad84da1c
("virtio-scsi: fixed virtio_scsi_ctx_check failed when detaching scsi
disk") to ensure that virtio_scsi_hotunplug() works while the guest
driver is submitting I/O.

Ensure virtio_scsi_hotunplug() is safe as follows:

1. qdev_simple_device_unplug_cb() -> qdev_unrealize() ->
   device_set_realized() calls qatomic_set(&dev->realized, false) so
   that future scsi_device_get() calls return NULL because they exclude
   SCSIDevices with realized=false.

   That means virtio-scsi will reject new I/O requests to this
   SCSIDevice with VIRTIO_SCSI_S_BAD_TARGET even while
   virtio_scsi_hotunplug() is still executing. We are protected against
   new requests!

2. Add a call to scsi_device_purge_requests() from scsi_unrealize() so
   that in-flight requests are cancelled synchronously. This ensures
   that no in-flight requests remain once qdev_simple_device_unplug_cb()

Thanks to these two conditions we don't need aio_disable_external()

Cc: Zhengui Li <lizhengui@huawei.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
 hw/scsi/scsi-disk.c   | 1 +
 hw/scsi/virtio-scsi.c | 3 ---
 2 files changed, 1 insertion(+), 3 deletions(-)

diff --git a/hw/scsi/scsi-disk.c b/hw/scsi/scsi-disk.c
index 97c9b1c8cd..e01bd84541 100644
--- a/hw/scsi/scsi-disk.c
+++ b/hw/scsi/scsi-disk.c
@@ -2522,6 +2522,7 @@ static void scsi_realize(SCSIDevice *dev, Error **errp)
 static void scsi_unrealize(SCSIDevice *dev)
+    scsi_device_purge_requests(dev, SENSE_CODE(RESET));
     del_boot_device_lchs(&dev->qdev, NULL);
diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 000961446c..a02f9233ec 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -1061,11 +1061,8 @@ static void virtio_scsi_hotunplug(HotplugHandler 
*hotplug_dev, DeviceState *dev,
     VirtIODevice *vdev = VIRTIO_DEVICE(hotplug_dev);
     VirtIOSCSI *s = VIRTIO_SCSI(vdev);
     SCSIDevice *sd = SCSI_DEVICE(dev);
-    AioContext *ctx = s->ctx ?: qemu_get_aio_context();
-    aio_disable_external(ctx);
     qdev_simple_device_unplug_cb(hotplug_dev, dev, errp);
-    aio_enable_external(ctx);
     if (s->ctx) {

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