Re: [PATCH v2] nvme-tcp: Check if request has started before processing it

[Sagi Grimberg]

> > > > I would also mention, that a crash is not even the scariest issue that
> > > > we can see here, because if the request happened to be reused we are
> > > > in the silent data corruption realm...
> > >
> > > If this does happen, I think we have to come up with some way to
> > > mitigate it. We're not utilizing the full 16 bits of the command_id, so
> > > maybe we can append something like a generation sequence number that can
> > > be checked for validity.
> >
> > That's actually a great idea. scsi needs unique tags so it encodes the
> > hwq in the upper 16 bits giving the actual tag the lower 16 bits which
> > is more than enough for a single queue. We can do the same with
> > a gencnt that will increment in both submission and completion and we
> > can validate against it.
> >
> > This will be useful for all transports, so maintaining it in
> > nvme_req(rq)->genctr and introducing a helper like:
> > rq = nvme_find_tag(tagset, cqe->command_id)
> > That will filter genctr, locate the request.
> >
> > Also:
> > nvme_validate_request_gen(rq, cqe->command_id) that would
> > compare against it.
> >
> >
> > And then a helper to set the command_id like:
> > cmd->common.command_id = nvme_request_command_id(rq)
> > that will both increment the genctr and build a command_id
> > from it.
> >
> > Thoughts?
>
> Well, that would require a modification to the CQE specification, no?
> fmds was not amused when I proposed that :-(

Why would that require a modification to the CQE? it's just using say
4 msbits of the command_id to a running sequence...