Re: [Qemu-devel] [PATCH 05/16] iothread: release AioContext around aio_poll

[Stefan Hajnoczi]

On Fri, Jan 15, 2016 at 04:12:08PM +0100, Paolo Bonzini wrote:
> @@ -120,9 +117,17 @@ Block layer code must therefore expect to run in an 
> IOThread and avoid using
>  old APIs that implicitly use the main loop.  See the "How to program for
>  IOThreads" above for information on how to do that.
>  
> -If main loop code such as a QMP function wishes to access a BlockDriverState 
> it
> -must first call aio_context_acquire(bdrv_get_aio_context(bs)) to ensure the
> -IOThread does not run in parallel.
> +If main loop code such as a QMP function wishes to access a BlockDriverState
> +it must first call aio_context_acquire(bdrv_get_aio_context(bs)) to ensure
> +that callbacks in the IOThread do not run in parallel.
> +
> +Code running in the monitor typically uses bdrv_drain() to ensure that
> +past requests from the guest are completed.  When a block device is
> +running in an IOThread, the IOThread can also process requests from
> +the guest (via ioeventfd).  These requests have to be blocked with
> +aio_disable_clients() before calling bdrv_drain().  You can then reenable
> +guest requests with aio_enable_clients() before finally releasing the
> +AioContext and completing the monitor command.

This is outdated.  Monitor commands do this:
bdrv_drained_begin() -> ... -> bdrv_drain() -> ... -> bdrv_drained_end()

> @@ -110,6 +111,8 @@ static void *test_acquire_thread(void *opaque)
>      qemu_mutex_lock(&data->start_lock);
>      qemu_mutex_unlock(&data->start_lock);
>  
> +    g_usleep(500000);

Sleep?

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