Re: [Qemu-devel] [PATCH v2 05/16] mirror: use bottom half to re-enter coroutine

[Fam Zheng <famz@redhat.com>]

On Wed, 03/16 15:16, Paolo Bonzini wrote:
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>         v1->v2: use aio_bh_new() [Fam]
> 
>  block/mirror.c | 24 ++++++++++++++++++++----
>  1 file changed, 20 insertions(+), 4 deletions(-)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index 9635fa8..2c7874d 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -72,6 +72,7 @@ typedef struct MirrorOp {
>      QEMUIOVector qiov;
>      int64_t sector_num;
>      int nb_sectors;
> +    QEMUBH *co_enter_bh;
>  } MirrorOp;
>  
>  static BlockErrorAction mirror_error_action(MirrorBlockJob *s, bool read,
> @@ -87,6 +88,18 @@ static BlockErrorAction mirror_error_action(MirrorBlockJob *s, bool read,
>      }
>  }
>  
> +static void mirror_bh_cb(void *opaque)
> +{
> +    MirrorOp *op = opaque;
> +    MirrorBlockJob *s = op->s;
> +
> +    qemu_bh_delete(op->co_enter_bh);
> +    g_free(op);
> +    if (s->waiting_for_io) {
> +        qemu_coroutine_enter(s->common.co, NULL);
> +    }
> +}
> +
>  static void mirror_iteration_done(MirrorOp *op, int ret)
>  {
>      MirrorBlockJob *s = op->s;
> @@ -117,11 +130,14 @@ static void mirror_iteration_done(MirrorOp *op, int ret)
>      }
>  
>      qemu_iovec_destroy(&op->qiov);
> -    g_free(op);
>  
> -    if (s->waiting_for_io) {
> -        qemu_coroutine_enter(s->common.co, NULL);
> -    }
> +    /* The I/O operation is not finished until the callback returns.
> +     * If we call qemu_coroutine_enter here, there is the possibility
> +     * of a deadlock when the coroutine calls bdrv_drained_begin.
> +     */
> +    op->co_enter_bh = aio_bh_new(bdrv_get_aio_context(s->target),
> +                                 mirror_bh_cb, op);
> +    qemu_bh_schedule(op->co_enter_bh);

I think we can conditionally create the BH only if s->waiting_for_io is true,
but the else branch is less important, so:

Reviewed-by: Fam Zheng <famz@redhat.com>