Re: [PATCH v5 05/21] virtio-scsi: stop using aio_disable_external() during unplug

[Kevin Wolf <kwolf@redhat.com>]

Am 04.05.2023 um 21:53 hat Stefan Hajnoczi geschrieben:
> 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(). It was 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. scsi_device_unrealize() already contains a call to

I think you mean scsi_qdev_unrealize(). Can be fixed while applying.

>    scsi_device_purge_requests() so that in-flight requests are cancelled
>    synchronously. This ensures that no in-flight requests remain once
>    qdev_simple_device_unplug_cb() returns.
> 
> Thanks to these two conditions we don't need aio_disable_external()
> anymore.
> 
> Cc: Zhengui Li <lizhengui@huawei.com>
> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> Reviewed-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Kevin