Currently NFSD (and the Linux NFS client) have but a single backchannel session slot. This serializes backchannel all operation to each client. Normally there isn't a lot of traffic on the backchannel, so increasing the backchannel slot count hasn't been a priority.
A to-do comment in fs/nfsd/nfs4callback.c mentions that NFSD should support multiple session slots.
It's not clear which clients might support multiple backchannel session slots, nor whether there is enough traffic on the backchannel to warrant more slots.
Currently NFSD (and the Linux NFS client) have but a single backchannel session slot. This serializes backchannel all operation to each client. Normally there isn't a lot of traffic on the backchannel, so increasing the backchannel slot count hasn't been a priority.
A to-do comment in fs/nfsd/nfs4callback.c mentions that NFSD should support multiple session slots.