Re: [PATCH v4 4/5] hw/virtio: generalise CHR_EVENT_CLOSED handling

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Nov 30, 2022 at 11:24:38AM +0000, Alex Bennée wrote:
> ..and use for both virtio-user-blk and virtio-user-gpio. This avoids
> the circular close by deferring shutdown due to disconnection until a
> later point.

I thought re-entrancy was already avoided by Patch 3?

> virtio-user-blk already had this mechanism in place so
> generalise it as a vhost-user helper function and use for both blk and
> gpio devices.
> 
> While we are at it we also fix up vhost-user-gpio to re-establish the
> event handler after close down so we can reconnect later.

I don't understand how the BH is supposed to help. Normally BHs are used
to defer processing while we're in a call stack of some depth. The
callers still require the resource we want to free, so we scheduled a
BH.

That's not the case here. The CHR_EVENT_CLOSED callback is just another
top-level event handler, not something that is called deep in a call
stack.

This BH approach isn't safe against nested event loops (something that
calls aio_poll() to wait), because now the BH can be invoked deep in a
call stack.

That said, if Patch 3 isn't enough and this patch fixes a problem, then
let's keep it for 7.2. It feels like a possibly incomplete solution and
maybe something that should be solved differently in 8.0.

> 
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
> ---
>  include/hw/virtio/vhost-user.h | 18 +++++++++
>  hw/block/vhost-user-blk.c      | 41 +++-----------------
>  hw/virtio/vhost-user-gpio.c    | 11 +++++-
>  hw/virtio/vhost-user.c         | 71 ++++++++++++++++++++++++++++++++++
>  4 files changed, 104 insertions(+), 37 deletions(-)
> 
> diff --git a/include/hw/virtio/vhost-user.h b/include/hw/virtio/vhost-user.h
> index c6e693cd3f..191216a74f 100644
> --- a/include/hw/virtio/vhost-user.h
> +++ b/include/hw/virtio/vhost-user.h
> @@ -68,4 +68,22 @@ bool vhost_user_init(VhostUserState *user, CharBackend *chr, Error **errp);
>   */
>  void vhost_user_cleanup(VhostUserState *user);
>  
> +/**
> + * vhost_user_async_close() - cleanup vhost-user post connection drop
> + * @d: DeviceState for the associated device (passed to callback)
> + * @chardev: the CharBackend associated with the connection
> + * @vhost: the common vhost device
> + * @cb: the user callback function to complete the clean-up
> + *
> + * This function is used to handle the shutdown of a vhost-user
> + * connection to a backend. We handle this centrally to make sure we
> + * do all the steps and handle potential races due to VM shutdowns.
> + * Once the connection is disabled we call a backhalf to ensure
> + */
> +typedef void (*vu_async_close_fn)(DeviceState *cb);
> +
> +void vhost_user_async_close(DeviceState *d,
> +                            CharBackend *chardev, struct vhost_dev *vhost,
> +                            vu_async_close_fn cb);
> +
>  #endif
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index 1177064631..aff4d2b8cb 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -369,17 +369,10 @@ static void vhost_user_blk_disconnect(DeviceState *dev)
>      vhost_user_blk_stop(vdev);
>  
>      vhost_dev_cleanup(&s->dev);
> -}
>  
> -static void vhost_user_blk_chr_closed_bh(void *opaque)
> -{
> -    DeviceState *dev = opaque;
> -    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> -    VHostUserBlk *s = VHOST_USER_BLK(vdev);
> -
> -    vhost_user_blk_disconnect(dev);
> +    /* Re-instate the event handler for new connections */
>      qemu_chr_fe_set_handlers(&s->chardev, NULL, NULL, vhost_user_blk_event,
> -                             NULL, opaque, NULL, true);
> +                             NULL, dev, NULL, true);
>  }
>  
>  static void vhost_user_blk_event(void *opaque, QEMUChrEvent event)
> @@ -398,33 +391,9 @@ static void vhost_user_blk_event(void *opaque, QEMUChrEvent event)
>          }
>          break;
>      case CHR_EVENT_CLOSED:
> -        if (!runstate_check(RUN_STATE_SHUTDOWN)) {
> -            /*
> -             * A close event may happen during a read/write, but vhost
> -             * code assumes the vhost_dev remains setup, so delay the
> -             * stop & clear.
> -             */
> -            AioContext *ctx = qemu_get_current_aio_context();
> -
> -            qemu_chr_fe_set_handlers(&s->chardev, NULL, NULL, NULL, NULL,
> -                    NULL, NULL, false);
> -            aio_bh_schedule_oneshot(ctx, vhost_user_blk_chr_closed_bh, opaque);
> -
> -            /*
> -             * Move vhost device to the stopped state. The vhost-user device
> -             * will be clean up and disconnected in BH. This can be useful in
> -             * the vhost migration code. If disconnect was caught there is an
> -             * option for the general vhost code to get the dev state without
> -             * knowing its type (in this case vhost-user).
> -             *
> -             * FIXME: this is sketchy to be reaching into vhost_dev
> -             * now because we are forcing something that implies we
> -             * have executed vhost_dev_stop() but that won't happen
> -             * until vhost_user_blk_stop() gets called from the bh.
> -             * Really this state check should be tracked locally.
> -             */
> -            s->dev.started = false;
> -        }
> +        /* defer close until later to avoid circular close */
> +        vhost_user_async_close(dev, &s->chardev, &s->dev,
> +                               vhost_user_blk_disconnect);
>          break;
>      case CHR_EVENT_BREAK:
>      case CHR_EVENT_MUX_IN:
> diff --git a/hw/virtio/vhost-user-gpio.c b/hw/virtio/vhost-user-gpio.c
> index be9be08b4c..b7b82a1099 100644
> --- a/hw/virtio/vhost-user-gpio.c
> +++ b/hw/virtio/vhost-user-gpio.c
> @@ -233,6 +233,8 @@ static int vu_gpio_connect(DeviceState *dev, Error **errp)
>      return 0;
>  }
>  
> +static void vu_gpio_event(void *opaque, QEMUChrEvent event);
> +
>  static void vu_gpio_disconnect(DeviceState *dev)
>  {
>      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> @@ -245,6 +247,11 @@ static void vu_gpio_disconnect(DeviceState *dev)
>  
>      vu_gpio_stop(vdev);
>      vhost_dev_cleanup(&gpio->vhost_dev);
> +
> +    /* Re-instate the event handler for new connections */
> +    qemu_chr_fe_set_handlers(&gpio->chardev,
> +                             NULL, NULL, vu_gpio_event,
> +                             NULL, dev, NULL, true);
>  }
>  
>  static void vu_gpio_event(void *opaque, QEMUChrEvent event)
> @@ -262,7 +269,9 @@ static void vu_gpio_event(void *opaque, QEMUChrEvent event)
>          }
>          break;
>      case CHR_EVENT_CLOSED:
> -        vu_gpio_disconnect(dev);
> +        /* defer close until later to avoid circular close */
> +        vhost_user_async_close(dev, &gpio->chardev, &gpio->vhost_dev,
> +                               vu_gpio_disconnect);
>          break;
>      case CHR_EVENT_BREAK:
>      case CHR_EVENT_MUX_IN:
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index abe23d4ebe..8f635844af 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -21,6 +21,7 @@
>  #include "qemu/error-report.h"
>  #include "qemu/main-loop.h"
>  #include "qemu/sockets.h"
> +#include "sysemu/runstate.h"
>  #include "sysemu/cryptodev.h"
>  #include "migration/migration.h"
>  #include "migration/postcopy-ram.h"
> @@ -2670,6 +2671,76 @@ void vhost_user_cleanup(VhostUserState *user)
>      user->chr = NULL;
>  }
>  
> +
> +typedef struct {
> +    vu_async_close_fn cb;
> +    DeviceState *dev;
> +    CharBackend *cd;
> +    struct vhost_dev *vhost;
> +} VhostAsyncCallback;
> +
> +static void vhost_user_async_close_bh(void *opaque)
> +{
> +    VhostAsyncCallback *data = opaque;
> +    struct vhost_dev *vhost = data->vhost;
> +
> +    /*
> +     * If the vhost_dev has been cleared in the meantime there is
> +     * nothing left to do as some other path has completed the
> +     * cleanup.
> +     */
> +    if (vhost->vdev) {
> +        data->cb(data->dev);
> +    }
> +
> +    g_free(data);
> +}
> +
> +/*
> + * We only schedule the work if the machine is running. If suspended
> + * we want to keep all the in-flight data as is for migration
> + * purposes.
> + */
> +void vhost_user_async_close(DeviceState *d,
> +                            CharBackend *chardev, struct vhost_dev *vhost,
> +                            vu_async_close_fn cb)
> +{
> +    if (!runstate_check(RUN_STATE_SHUTDOWN)) {
> +        /*
> +         * A close event may happen during a read/write, but vhost
> +         * code assumes the vhost_dev remains setup, so delay the
> +         * stop & clear.
> +         */
> +        AioContext *ctx = qemu_get_current_aio_context();
> +        VhostAsyncCallback *data = g_new0(VhostAsyncCallback, 1);
> +
> +        /* Save data for the callback */
> +        data->cb = cb;
> +        data->dev = d;
> +        data->cd = chardev;
> +        data->vhost = vhost;
> +
> +        /* Disable any further notifications on the chardev */
> +        qemu_chr_fe_set_handlers(chardev,
> +                                 NULL, NULL, NULL, NULL, NULL, NULL,
> +                                 false);
> +
> +        aio_bh_schedule_oneshot(ctx, vhost_user_async_close_bh, data);
> +
> +        /*
> +         * Move vhost device to the stopped state. The vhost-user device
> +         * will be clean up and disconnected in BH. This can be useful in
> +         * the vhost migration code. If disconnect was caught there is an
> +         * option for the general vhost code to get the dev state without
> +         * knowing its type (in this case vhost-user).
> +         *
> +         * Note if the vhost device is fully cleared by the time we
> +         * execute the bottom half we won't continue with the cleanup.
> +         */
> +        vhost->started = false;
> +    }
> +}
> +
>  static int vhost_user_dev_start(struct vhost_dev *dev, bool started)
>  {
>      if (!virtio_has_feature(dev->protocol_features,
> -- 
> 2.34.1
> 

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