Re: [Qemu-devel] [PATCH 1/3] block: add BlockBackend->in_flight counter

[Paolo Bonzini <pbonzini@redhat.com>]

On 08/02/2018 18:18, Stefan Hajnoczi wrote:
> +    BlockDriverState *bs = blk_bs(blk);
> +
> +    if (bs) {
> +        bdrv_drained_begin(bs);
> +    }
> +
> +    /* We may have aio requests like -ENOMEDIUM in flight */
> +    while (atomic_mb_read(&blk->in_flight) > 0) {
> +        aio_poll(blk_get_aio_context(blk), true);
> +    }
> +
> +    if (bs) {
> +        bdrv_drained_end(bs);

This only works if you are in the default AioContext, otherwise you can
run handlers for one AioContexts in two threads.

bdrv_dec_in_flight uses bdrv_wakeup and BDRV_POLL_WHILE to ensure that
this doesn't happen, so there would be more code that you have to copy
into block-backend.c.

Paolo