Re: [PATCH 06/12] qcow2: Fix open with 'file' in iothread

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, May 25, 2023 at 02:47:07PM +0200, Kevin Wolf wrote:
> qcow2_open() doesn't work correctly when opening the 'file' child moves
> bs to an iothread, for several reasons:
> 
> - It uses BDRV_POLL_WHILE() to wait for the qcow2_open_entry()
>   coroutine, which involves dropping the AioContext lock for bs when it
>   is not in the main context - but we don't hold it, so this crashes.
> 
> - It runs the qcow2_open_entry() coroutine in the current thread instead
>   of the new AioContext of bs.
> 
> - qcow2_open_entry() doesn't notify the main loop when it's done.
> 
> This patches fixes these issues around delegating work to a coroutine.
> Temporarily dropping the main AioContext lock is not necessary because
> we know we run in the main thread.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block/qcow2.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index b00b4e7575..7f3948360d 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -1904,6 +1904,8 @@ static void coroutine_fn qcow2_open_entry(void *opaque)
>      qoc->ret = qcow2_do_open(qoc->bs, qoc->options, qoc->flags, true,
>                               qoc->errp);
>      qemu_co_mutex_unlock(&s->lock);
> +
> +    aio_wait_kick();
>  }
>  
>  static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
> @@ -1929,8 +1931,10 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
>  
>      assert(!qemu_in_coroutine());
>      assert(qemu_get_current_aio_context() == qemu_get_aio_context());
> -    qemu_coroutine_enter(qemu_coroutine_create(qcow2_open_entry, &qoc));
> -    BDRV_POLL_WHILE(bs, qoc.ret == -EINPROGRESS);
> +
> +    aio_co_enter(bdrv_get_aio_context(bs),
> +                 qemu_coroutine_create(qcow2_open_entry, &qoc));
> +    AIO_WAIT_WHILE_UNLOCKED(NULL, qoc.ret == -EINPROGRESS);

Want to update the doc comment for bdrv_open_file_child() with a warning
that @parent's AioContext can change?

Otherwise:
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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