Re: [Qemu-devel] [PATCH 4/4] migration: add missed aio_context_acquire into HMP snapshot code

[Juan Quintela]

"Denis V. Lunev" <address@hidden> wrote:
> aio_context should be locked in the similar way as was done in QMP
> snapshot creation in the other case there are a lot of possible
> troubles if native AIO mode is enabled for disk.
>
> - the command can hang (HMP thread) with missed wakeup (the operation is
>   actually complete)
>     io_submit
>     ioq_submit
>     laio_submit
>     raw_aio_submit
>     raw_aio_readv
>     bdrv_co_io_em
>     bdrv_co_readv_em
>     bdrv_aligned_preadv
>     bdrv_co_do_preadv
>     bdrv_co_do_readv
>     bdrv_co_readv
>     qcow2_co_readv
>     bdrv_aligned_preadv
>     bdrv_co_do_pwritev
>     bdrv_rw_co_entry
>
> - QEMU can assert in coroutine re-enter
>     __GI_abort
>     qemu_coroutine_enter
>     bdrv_co_io_em_complete
>     qemu_laio_process_completion
>     qemu_laio_completion_bh
>     aio_bh_poll
>     aio_dispatch
>     aio_poll
>     iothread_run
>
> qemu_fopen_bdrv and bdrv_fclose are used in real snapshot operations only
> along with block drivers. This change should influence only HMP snapshot
> operations.
>
> AioContext lock is reqursive. Thus nested locking should not be a problem.
>
> Signed-off-by: Denis V. Lunev <address@hidden>
> CC: Stefan Hajnoczi <address@hidden>
> CC: Paolo Bonzini <address@hidden>
> CC: Juan Quintela <address@hidden>
> CC: Amit Shah <address@hidden>

Reviewed-by: Juan Quintela <address@hidden>

Should this one go through the block layer?  I guess that the block
layer, but otherwise, I will get it.


> -    return bdrv_flush(opaque);

> +    BlockDriverState *bs = (BlockDriverState *)opaque;

Cast not needed.

BlockDriverState * bs = opaque;

is even better.

Thanks, Juan.