Re: [Xen-devel] [PATCH 2/2] xenbus: req->err should be updated before req->state

[Julien Grall]

Hi,

On 03/03/2020 01:58, Dongli Zhang wrote:
> This patch adds the barrier to guarantee that req->err is always updated
> before req->state.
>
> Otherwise, read_reply() would not return ERR_PTR(req->err) but
> req->body, when process_writes()->xb_write() is failed.

The memory barrier below looks good. However, as mentionned in patch #1, 
barrier() is not the correct barrier to pair with virt_wmb().

> Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> ---
>   drivers/xen/xenbus/xenbus_comms.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c
> index 852ed161fc2a..eb5151fc8efa 100644
> --- a/drivers/xen/xenbus/xenbus_comms.c
> +++ b/drivers/xen/xenbus/xenbus_comms.c
> @@ -397,6 +397,8 @@ static int process_writes(void)
>   	if (state.req->state == xb_req_state_aborted)
>   		kfree(state.req);
>   	else {
> +		/* write err, then update state */
> +		virt_wmb();
>   		state.req->state = xb_req_state_got_reply;
>   		wake_up(&state.req->wq);
>   	}

Cheers,

--
Julien Grall