Re: [Qemu-block] [Qemu-devel] [PATCH 2/3] block: use BDRV_POLL_WHILE() in bdrv_rw_vmstate()

[Eric Blake]

On 05/17/2017 12:09 PM, Stefan Hajnoczi wrote:
> Calling aio_poll() directly may have been fine previously, but this is
> the future, man!

lol

>  The difference between an aio_poll() loop and
> BDRV_POLL_WHILE() is that BDRV_POLL_WHILE() releases the AioContext
> around aio_poll().
> 
> This allows the IOThread to run fd handlers or BHs to complete the
> request.  Failure to release the AioContext causes deadlocks.
> 
> Using BDRV_POLL_WHILE() partially fixes a 'savevm' hang with -object
> iothread.

I'm surprised at how many separate hangs we actually had!

> 
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  block/io.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)

Reviewed-by: Eric Blake <address@hidden>

> 
> diff --git a/block/io.c b/block/io.c
> index cc56e90..f0041cd 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -2031,9 +2031,7 @@ bdrv_rw_vmstate(BlockDriverState *bs, QEMUIOVector 
> *qiov, int64_t pos,
>          Coroutine *co = qemu_coroutine_create(bdrv_co_rw_vmstate_entry, 
> &data);
>  
>          bdrv_coroutine_enter(bs, co);
> -        while (data.ret == -EINPROGRESS) {
> -            aio_poll(bdrv_get_aio_context(bs), true);
> -        }
> +        BDRV_POLL_WHILE(bs, data.ret == -EINPROGRESS);
>          return data.ret;
>      }

Do we have other culprits (not necessarily for vmsave, but for other
situations), where we should be using BDRV_POLL_WHILE in separate
patches? For example, a quick grep show that at least hw/9pfs/9p.c makes
a direct call to aio_poll(,true) in a while loop.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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