Re: [PATCH 2/3] block-backend: Update ctx immediately after root

[Kevin Wolf]

Am 23.09.2022 um 14:52 hat Hanna Reitz geschrieben:
> blk_get_aio_context() asserts that blk->ctx is always equal to the root
> BDS's context (if there is a root BDS).  Therefore,
> blk_do_set_aio_context() must update blk->ctx immediately after the root
> BDS's context has changed.
> 
> Without this patch, the next patch would break iotest 238, because
> bdrv_drained_begin() (called by blk_do_set_aio_context()) may then
> invoke bdrv_child_get_parent_aio_context() on the root child, i.e.
> blk_get_aio_context().  However, by this point, blk->ctx would not have
> been updated and thus differ from the root node's context.  This patch
> fixes that.
> 
> Signed-off-by: Hanna Reitz <hreitz@redhat.com>
> ---
>  block/block-backend.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/block/block-backend.c b/block/block-backend.c
> index d4a5df2ac2..abdb5ff5af 100644
> --- a/block/block-backend.c
> +++ b/block/block-backend.c
> @@ -2156,6 +2156,7 @@ static int blk_do_set_aio_context(BlockBackend *blk, 
> AioContext *new_context,
>                  return ret;
>              }
>          }
> +        blk->ctx = new_context;
>          if (tgm->throttle_state) {
>              bdrv_drained_begin(bs);
>              throttle_group_detach_aio_context(tgm);
> @@ -2164,9 +2165,10 @@ static int blk_do_set_aio_context(BlockBackend *blk, 
> AioContext *new_context,
>          }
>  
>          bdrv_unref(bs);
> +    } else {
> +        blk->ctx = new_context;
>      }
>  
> -    blk->ctx = new_context;
>      return 0;
>  }

Makes sense. Maybe in the first branch a comment wouldn't hurt (like
"make blk->ctx consistent with the root node again before any other
operations like drain").

Reviewed-by: Kevin Wolf <kwolf@redhat.com>