Re: [Qemu-block] [PATCH for-2.9] thread-pool: add missing qemu_bh_cancel in completion function

[Paolo Bonzini]

On 16/03/2017 17:02, Peter Lieven wrote:
> commit 3c80ca15 fixed a deadlock scenarion with nested aio_poll invocations.
> 
> However, the rescheduling of the completion BH introcuded unnecessary spinning
> in the main-loop. On very fast file backends this can even lead to the
> "WARNING: I/O thread spun for 1000 iterations" message popping up.
> 
> Callgrind reports about 3-4% less instructions with this patch running
> qemu-img bench on a ramdisk based VMDK file.
> 
> Fixes: 3c80ca158c96ff902a30883a8933e755988948b1
> Cc: address@hidden
> Signed-off-by: Peter Lieven <address@hidden>
> ---
>  util/thread-pool.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/util/thread-pool.c b/util/thread-pool.c
> index ce6cd30..610646d 100644
> --- a/util/thread-pool.c
> +++ b/util/thread-pool.c
> @@ -188,6 +188,13 @@ restart:
>              aio_context_release(pool->ctx);
>              elem->common.cb(elem->common.opaque, elem->ret);
>              aio_context_acquire(pool->ctx);
> +
> +            /* We can safely cancel the completion_bh here regardless of 
> someone
> +             * else having scheduled it meanwhile because we reenter the
> +             * completion function anyway (goto restart).
> +             */
> +            qemu_bh_cancel(pool->completion_bh);
> +
>              qemu_aio_unref(elem);
>              goto restart;
>          } else {
> 

Right, this is the same thing that block/linux-aio.c does.

Paolo