Re: [PATCH 0/2] block-backend: process I/O in the current AioContext

[Kevin Wolf <kwolf@redhat.com>]

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