Re: WARN_ON added to rpc_create()

[bfields@fieldses.org (J. Bruce Fields)]

On Wed, Aug 03, 2016 at 11:27:47AM -0400, Chuck Lever wrote:
> Hi Bruce-
> 
> I see that commit 39a9beab5acb83176e8b9a4f0778749a09341f1f
> Author:     J. Bruce Fields <bfields@redhat.com>
> AuthorDate: Tue May 17 12:38:21 2016 -0400
> 
>     rpc: share one xps between all backchannels
> 
> has added this piece of code:
> 
> @@ -452,10 +452,20 @@ static struct rpc_clnt *rpc_create_xprt(struct rpc_create_args *args,
>         struct rpc_clnt *clnt = NULL;
>         struct rpc_xprt_switch *xps;
>  
> -       xps = xprt_switch_alloc(xprt, GFP_KERNEL);
> -       if (xps == NULL) {
> -               xprt_put(xprt);
> -               return ERR_PTR(-ENOMEM);
> +       if (args->bc_xprt && args->bc_xprt->xpt_bc_xps) {
> +               WARN_ON(args->protocol != XPRT_TRANSPORT_BC_TCP);
> +               xps = args->bc_xprt->xpt_bc_xps;
> +               xprt_switch_get(xps);
> +       } else {
> 
> 
> the WARN_ON here fires on the server whenever I use NFSv4.1 on RDMA.
> 
> Can you say why it was added? Is there something RPC/RDMA needs to
> do to make the code safe?

What is args->protocol in this case?

Digging around...  OK, I missed that BC_TCP and BC_RDMA were defined as
OR's of an XPRT_TRANSPORT_BC bit with the identifier of the underlying
transport.  That makes sense.

So, I should have just used XPRT_TRANSPORT_BC there--I think all I meant
was "is this a backchannel".

Does that fix the problem?

--b.