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

[Fam Zheng]

On Tue, 02/16 18:56, Paolo Bonzini wrote:
> mirror is calling bdrv_drain from an AIO callback---more precisely,
> the bdrv_drain happens far away from the AIO callback, in the coroutine that
> the AIO callback enters.
> 
> This used to be okay because bdrv_drain more or less tried to guess
> when all AIO callbacks were done; however it will cause a deadlock once
> bdrv_drain really checks for AIO callbacks to be complete.  The situation
> here is admittedly underdefined, and Stefan pointed out that the same
> solution is found in many other places in the QEMU block layer, therefore
> I think this workaround is acceptable.
> 
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
>  block/mirror.c | 23 +++++++++++++++++++----
>  1 file changed, 19 insertions(+), 4 deletions(-)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index 2c0edfa..793c20c 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -71,6 +71,7 @@ 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,
> @@ -86,6 +87,18 @@ 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;
> @@ -116,11 +129,13 @@ 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 = qemu_bh_new(mirror_bh_cb, op);

Shouldn't this be aio_bh_new()?

Fam

> +    qemu_bh_schedule(op->co_enter_bh);
>  }
>  
>  static void mirror_write_complete(void *opaque, int ret)
> -- 
> 2.5.0
> 
> 
>