Re: svc_xprt_put is no longer BH-safe

["J. Bruce Fields" <bfields@fieldses.org>]

On Tue, Nov 08, 2016 at 04:05:54PM -0500, Chuck Lever wrote:
> 
> > On Nov 8, 2016, at 3:20 PM, J. Bruce Fields <bfields@fieldses.org> wrote:
> > 
> > On Tue, Nov 08, 2016 at 03:13:13PM -0500, Chuck Lever wrote:
> >> 
> >>> On Nov 8, 2016, at 3:03 PM, J. Bruce Fields <bfields@fieldses.org> wrote:
> >>> 
> >>> On Sat, Oct 29, 2016 at 12:21:03PM -0400, Chuck Lever wrote:
> >>>> Hi Bruce-
> >>> 
> >>> Sorry for the slow response!
> >>> 
> >>> ...
> >>>> In commit 39a9beab5acb83176e8b9a4f0778749a09341f1f ('rpc: share one xps between
> >>>> all backchannels') you added:
> >>>> 
> >>>> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> >>>> index f5572e3..4f01f63 100644
> >>>> --- a/net/sunrpc/svc_xprt.c
> >>>> +++ b/net/sunrpc/svc_xprt.c
> >>>> @@ -136,6 +136,8 @@ static void svc_xprt_free(struct kref *kref)
> >>>>       /* See comment on corresponding get in xs_setup_bc_tcp(): */
> >>>>       if (xprt->xpt_bc_xprt)
> >>>>               xprt_put(xprt->xpt_bc_xprt);
> >>>> +       if (xprt->xpt_bc_xps)
> >>>> +               xprt_switch_put(xprt->xpt_bc_xps);
> >>>>       xprt->xpt_ops->xpo_free(xprt);
> >>>>       module_put(owner);
> >>>> }
> >>>> 
> >>>> svc_xprt_free() is invoked by svc_xprt_put(). svc_xprt_put() is called
> >>>> from svc_rdma in soft IRQ context (eg. svc_rdma_wc_receive).
> >>> 
> >>> Is that necessary?  I wonder why the svcrdma code seems to be taking so
> >>> many of its own references on svc_xprts.
> >> 
> >> The idea is to keep the xprt around while Work Requests (I/O) are running,
> >> so that the xprt is guaranteed to be there during work completions. The
> >> completion handlers (where svc_xprt_put is often invoked) run in soft IRQ
> >> context.
> >> 
> >> It's simple to change completions to use a Work Queue instead, but testing
> >> so far shows that will result in a performance loss. I'm still studying it.
> >> 
> >> Is there another way to keep the xprt's ref count boosted while I/O is
> >> going on?
> > 
> > Why do you need the svc_xprt in the completion?
> 
> 1. The svc_xprt contains the svcrdma_xprt, which contains the Send Queue
> accounting mechanism. SQ accounting has to be updated for each completion:
> the completion indicates that the SQ entry used by this Work Request is
> now available, and that other callers waiting for enough SQEs can go ahead.
> 
> 2. When handling a Receive completion, the incoming RPC message is enqueued
> on the svc_xprt via svc_xprt_enqueue, unless RDMA Reads are needed.
> 
> 3. When handling a Read completion, the incoming RPC message is enqueued on
> the svc_xprt via svc_xprt_enqueue.
> 
> 
> > Can the xpo_detach method wait for any pending completions?
> 
> So the completions would call wake_up on a waitqueue, or use a kernel
> completion? That sounds more expensive than managing an atomic reference
> count.
> 
> I suppose some other reference count could be used to trigger a work item
> that would invoke xpo_detach.
> 
> But based on your comments, then, svc_xprt_put() was never intended to be
> BH-safe.

I'm not sure what was intended.  It doesn't look to me like any other
callers require it.

--b.

> 
> 
> > --b.
> > 
> >> 
> >> 
> >>>> However, xprt_switch_put() takes a spin lock (xps_lock) which is locked
> >>>> everywhere without disabling BHs.
> >>>> 
> >>>> It looks to me like 39a9beab5acb makes svc_xprt_put() no longer BH-safe?
> >>>> Not sure if svc_xprt_put() was intended to be BH-safe beforehand.
> >>>> 
> >>>> Maybe xprt_switch_put() could be invoked in ->xpo_free, but that seems
> >>>> like a temporary solution.
> >>> 
> >>> Since xpo_free is also called from svc_xprt_put that doesn't sound like
> >>> it would change anything.  Or do we not trunk over RDMA for some reason?
> >> 
> >> It's quite desirable to trunk NFS/RDMA on multiple connections, and it
> >> should work just like it does for TCP, but so far it's not been tested.
> 
> 
> --
> Chuck Lever
> 
>