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

[Stefan Hajnoczi]

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

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