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Subject: Re: [Xen-devel] [PATCH 2/2] xenbus: req->err should be updated before
 req->state
To:     Dongli Zhang <dongli.zhang@oracle.com>,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
Cc:     jgross@suse.com, boris.ostrovsky@oracle.com,
        sstabellini@kernel.org, joe.jin@oracle.com
References: <20200303015859.18813-1-dongli.zhang@oracle.com>
 <20200303015859.18813-2-dongli.zhang@oracle.com>
From:   Julien Grall <julien@xen.org>
Message-ID: <4c2594c8-9146-fbd9-6074-9c8366afb391@xen.org>
Date:   Tue, 3 Mar 2020 10:11:59 +0000
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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@oracle.com>
> ---
>   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