Switch blk_aio_*() APIs over to multi-queue by using
qemu_get_current_aio_context() instead of blk_get_aio_context(). This change
will allow devices to process I/O in multiple IOThreads in the future.

Stefan Hajnoczi (2):
  block-backend: process I/O in the current AioContext
  block-backend: process zoned requests in the current AioContext

 block/block-backend.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

-- 
2.41.0

Am 15.08.2023 um 18:05 hat Stefan Hajnoczi geschrieben:
> Switch blk_aio_*() APIs over to multi-queue by using
> qemu_get_current_aio_context() instead of blk_get_aio_context(). This change
> will allow devices to process I/O in multiple IOThreads in the future.

Both code paths still use blk_aio_em_aiocb_info, which is:

    static AioContext *blk_aio_em_aiocb_get_aio_context(BlockAIOCB *acb_)
    {
        BlkAioEmAIOCB *acb = container_of(acb_, BlkAioEmAIOCB, common);

        return blk_get_aio_context(acb->rwco.blk);
    }

    static const AIOCBInfo blk_aio_em_aiocb_info = {
        .aiocb_size         = sizeof(BlkAioEmAIOCB),
        .get_aio_context    = blk_aio_em_aiocb_get_aio_context,
    };

.get_aio_context() is called by bdrv_aio_cancel(), which already looks
wrong before this patch because in theory it can end up polling the
AioContext of a different thread. After this patch, .get_aio_context()
doesn't even necessarily return the AioContext that runs the request any
more.

The only thing that might save us is that I can't find any device that
both supports iothreads and calls bdrv_aio_cancel(). But we shouldn't
rely on that.

Maybe the solution is to just remove .get_aio_context altogether and use
AIO_WAIT_WHILE(NULL, ...) in bdrv_aio_cancel().

Kevin

On Fri, Aug 18, 2023 at 05:24:22PM +0200, Kevin Wolf wrote:
> Am 15.08.2023 um 18:05 hat Stefan Hajnoczi geschrieben:
> > Switch blk_aio_*() APIs over to multi-queue by using
> > qemu_get_current_aio_context() instead of blk_get_aio_context(). This change
> > will allow devices to process I/O in multiple IOThreads in the future.
> 
> Both code paths still use blk_aio_em_aiocb_info, which is:
> 
>     static AioContext *blk_aio_em_aiocb_get_aio_context(BlockAIOCB *acb_)
>     {
>         BlkAioEmAIOCB *acb = container_of(acb_, BlkAioEmAIOCB, common);
> 
>         return blk_get_aio_context(acb->rwco.blk);
>     }
> 
>     static const AIOCBInfo blk_aio_em_aiocb_info = {
>         .aiocb_size         = sizeof(BlkAioEmAIOCB),
>         .get_aio_context    = blk_aio_em_aiocb_get_aio_context,
>     };
> 
> .get_aio_context() is called by bdrv_aio_cancel(), which already looks
> wrong before this patch because in theory it can end up polling the
> AioContext of a different thread. After this patch, .get_aio_context()
> doesn't even necessarily return the AioContext that runs the request any
> more.
> 
> The only thing that might save us is that I can't find any device that
> both supports iothreads and calls bdrv_aio_cancel(). But we shouldn't
> rely on that.
> 
> Maybe the solution is to just remove .get_aio_context altogether and use
> AIO_WAIT_WHILE(NULL, ...) in bdrv_aio_cancel().

I will remove AIOCBInfo.get_aio_context in v2.

Stefan