Re: [Qemu-devel] [Qemu-block] [PATCH RFC 04/16] block: Propagate BLK_PERM_AIO_CONTEXT_CHANGE down the graph

[Stefan Hajnoczi]

On Tue, Mar 21, 2017 at 11:16:23AM +0800, Fam Zheng wrote:
> @@ -1713,21 +1714,22 @@ void bdrv_format_default_perms(BlockDriverState *bs, 
> BdrvChild *c,
>          perm |= BLK_PERM_CONSISTENT_READ;
>          shared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
>      } else {
> -        /* We want consistent read from backing files if the parent needs it.
> +        /* We want consistent read and aio context change from backing files 
> if
> +         * the parent needs it.
>           * No other operations are performed on backing files. */
> -        perm &= BLK_PERM_CONSISTENT_READ;
> +        perm &= BLK_PERM_CONSISTENT_READ | BLK_PERM_AIO_CONTEXT_CHANGE;
>  
> -        /* If the parent can deal with changing data, we're okay with a
> +        /* If the parent can deal with changing aio context, we're okay too;
> +         * If the parent can deal with changing data, we're okay with a
>           * writable and resizable backing file. */
>          /* TODO Require !(perm & BLK_PERM_CONSISTENT_READ), too? */
> +        shared &= BLK_PERM_AIO_CONTEXT_CHANGE | BLK_PERM_WRITE;
>          if (shared & BLK_PERM_WRITE) {
> -            shared = BLK_PERM_WRITE | BLK_PERM_RESIZE;
> -        } else {
> -            shared = 0;
> +            shared |= BLK_PERM_WRITE | BLK_PERM_RESIZE;

We already have BLK_PERM_WRITE so we're just adding BLK_PERM_RESIZE.
The following is clearer:

  shared |= BLK_PERM_RESIZE;

>          }
>  
>          shared |= BLK_PERM_CONSISTENT_READ | BLK_PERM_GRAPH_MOD |
> -                  BLK_PERM_WRITE_UNCHANGED;
> +                  BLK_PERM_WRITE_UNCHANGED | BLK_PERM_AIO_CONTEXT_CHANGE;

Why was shared &= BLK_PERM_AIO_CONTEXT_CHANGE necessary above if we
unconditionally OR it here?

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