Re: [Qemu-devel] [RFC 1/5] block/nbd: Fix hang in .bdrv_close()

[Kevin Wolf <kwolf@redhat.com>]

Am 11.07.2019 um 21:58 hat Max Reitz geschrieben:
> When nbd_close() is called from a coroutine, the connection_co never
> gets to run, and thus nbd_teardown_connection() hangs.
> 
> This is because aio_co_enter() only puts the connection_co into the main
> coroutine's wake-up queue, so this main coroutine needs to yield and
> reschedule itself to let the connection_co run.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/nbd.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/block/nbd.c b/block/nbd.c
> index 81edabbf35..b83b6cd43e 100644
> --- a/block/nbd.c
> +++ b/block/nbd.c
> @@ -135,7 +135,17 @@ static void nbd_teardown_connection(BlockDriverState *bs)
>      qio_channel_shutdown(s->ioc,
>                           QIO_CHANNEL_SHUTDOWN_BOTH,
>                           NULL);
> -    BDRV_POLL_WHILE(bs, s->connection_co);
> +
> +    if (qemu_in_coroutine()) {
> +        /* Let our caller poll and just yield until connection_co is done */
> +        while (s->connection_co) {
> +            aio_co_schedule(qemu_get_current_aio_context(),
> +                            qemu_coroutine_self());
> +            qemu_coroutine_yield();
> +        }

Isn't this busy waiting? Why not let s->connection_co wake us up when
it's about to terminate instead of immediately rescheduling ourselves?

> +    } else {
> +        BDRV_POLL_WHILE(bs, s->connection_co);
> +    }
>  
>      nbd_client_detach_aio_context(bs);
>      object_unref(OBJECT(s->sioc));

Kevin