nfs stalls over loopback interface (no sk_data_ready events?)

[Jeff Layton <jeff.layton-7I+n7zu2hftEKMMhf/gKZA@public.gmane.org>]

Sending this to netdev since I think I've now determined that this is
not a NFS specific problem. Recently Christoph mentioned that he was
seeing stalls when running xfstests generic/075 test on NFS over the
loopback interface with v3.18-rc1-ish kernel.

The configuration in this case is the nfs server and client on same box
communicating over the lo interface.

Here's are tracepoints from a typical request as it's supposed to work:

       mount.nfs-906   [002] ...1 22711.996969: xprt_transmit: xprt=0xffff8800ce961000 xid=0xa8a34513 status=0
            nfsd-678   [000] ...1 22711.997082: svc_recv: rq_xid=0xa8a34513 status=164
            nfsd-678   [000] ..s8 22711.997185: xprt_lookup_rqst: xprt=0xffff8800ce961000 xid=0xa8a34513 status=0
            nfsd-678   [000] ..s8 22711.997186: xprt_complete_rqst: xprt=0xffff8800ce961000 xid=0xa8a34513 status=140
            nfsd-678   [000] ...1 22711.997236: svc_send: rq_xid=0xa8a34513 dropme=0 status=144
            nfsd-678   [000] ...1 22711.997236: svc_process: rq_xid=0xa8a34513 dropme=0 status=144

...basically, we send a request to the server. Server picks it up and
sends the reply, and then the client IDs that reply and processes it.
This runs along just fine for ~ a minute or so. At some point, the
client stops seeing replies come in:

     kworker/2:2-107   [002] ...1 22741.696070: xprt_transmit: xprt=0xffff8800ce961000 xid=0xc3a84513 status=0
            nfsd-678   [002] .N.1 22741.696917: svc_recv: rq_xid=0xc3a84513 status=208
            nfsd-678   [002] ...1 22741.699890: svc_send: rq_xid=0xc3a84513 dropme=0 status=262252
            nfsd-678   [002] ...1 22741.699891: svc_process: rq_xid=0xc3a84513 dropme=0 status=262252


...a bit more tracepoint work seems to show that we just stop getting
sk_data_ready callbacks on the socket at all. I'm not terribly familiar
with the lower-level socket code, so I figured I'd email here and ask...

Anyone have insight into why this might be happening?

Thanks,
-- 
Jeff Layton <jlayton-7I+n7zu2hftEKMMhf/gKZA@public.gmane.org>
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