Re: [PATCH v7 12/13] block: Add bdrv_co_move_to_aio_context()

[Stefan Hajnoczi]

On Wed, Sep 09, 2020 at 05:11:48PM +0200, Kevin Wolf wrote:
> Add a function to move the current coroutine to the AioContext of a
> given BlockDriverState.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  include/block/block.h |  6 ++++++
>  block.c               | 10 ++++++++++
>  2 files changed, 16 insertions(+)
> 
> diff --git a/include/block/block.h b/include/block/block.h
> index 981ab5b314..80ab322f11 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -626,6 +626,12 @@ bool bdrv_debug_is_suspended(BlockDriverState *bs, const 
> char *tag);
>   */
>  AioContext *bdrv_get_aio_context(BlockDriverState *bs);
>  
> +/**
> + * Move the current coroutine to the AioContext of @bs and return the old
> + * AioContext of the coroutine.
> + */
> +AioContext *coroutine_fn bdrv_co_move_to_aio_context(BlockDriverState *bs);

I'm not sure this function handles all cases:
1. Being called without the BQL (i.e. from an IOThread).
2. Being called while a device stops using its IOThread.

The races that come to mind are fetching the AioContext for bs and then
scheduling a BH. The BH is executed later on by the event loop. There
might be cases where the AioContext for bs is updated before the BH
runs.

I didn't investigate these cases but wanted to mention them in case you
want to add doc comments about when this function can be used or if
you'd like to verify them yourself.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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