On 9/13/19 6:56 AM, Sergio Lopez wrote:
> virtio_notify_config() needs to acquire the global mutex, which isn't
> allowed from an iothread, and may lead to a deadlock like this:
>
> - main thead
> * Has acquired: qemu_global_mutex.
> * Is trying the acquire: iothread AioContext lock via
> AIO_WAIT_WHILE (after aio_poll).
>
> - iothread
> * Has acquired: AioContext lock.
> * Is trying to acquire: qemu_global_mutex (via
> virtio_notify_config->prepare_mmio_access).
>
> If virtio_blk_resize() is called from an iothread, schedule
> virtio_notify_config() to be run in the main context BH.
>
> Signed-off-by: Sergio Lopez <s...@redhat.com>
> ---
> Changelog
>
> v2:
> - Use aio_bh_schedule_oneshot instead of scheduling a coroutine
> (thanks Kevin Wolf).
> - Switch from RFC to v2 patch.
> ---
> hw/block/virtio-blk.c | 21 ++++++++++++++++++++-
> 1 file changed, 20 insertions(+), 1 deletion(-)
>
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index 18851601cb..669dc60f5b 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -16,6 +16,7 @@
> #include "qemu/iov.h"
> #include "qemu/module.h"
> #include "qemu/error-report.h"
> +#include "qemu/main-loop.h"
> #include "trace.h"
> #include "hw/block/block.h"
> #include "hw/qdev-properties.h"
> @@ -1086,11 +1087,29 @@ static int virtio_blk_load_device(VirtIODevice *vdev,
> QEMUFile *f,
> return 0;
> }
>
> +static void virtio_resize_cb(void *opaque)
> +{
> + VirtIODevice *vdev = opaque;
> +
> + assert(qemu_get_current_aio_context() == qemu_get_aio_context());
> + virtio_notify_config(vdev);
> +}
> +
> static void virtio_blk_resize(void *opaque)
> {
> VirtIODevice *vdev = VIRTIO_DEVICE(opaque);
>
> - virtio_notify_config(vdev);
> + if (qemu_get_current_aio_context() != qemu_get_aio_context()) {
> + /*
> + * virtio_notify_config() needs to acquire the global mutex,
> + * so it can't be called from an iothread. Instead, schedule
> + * it to be run in the main context BH.
> + */
> + aio_bh_schedule_oneshot(qemu_get_aio_context(),
> + virtio_resize_cb, vdev);
> + } else {
> + virtio_notify_config(vdev);
> + }
> }
>
> static const BlockDevOps virtio_block_ops = {
>
Do we need to bother to check our current context before firing off the
oneshot? It's simpler to just fire off the oneshot. I can't imagine that
resize is a hot path that needs to worry about being that efficient.
Well, I guess it doesn't hurt either.
Reviewed-by: John Snow <js...@redhat.com>
