RE: [PATCH v3 05/11] xprtrdma: Do not wait if ib_post_send() fails

["Steve Wise" <swise-7bPotxP6k4+P2YhJcF5u+vpXobYPEAuW@public.gmane.org>]

> >>>>>>>>>> Moving the QP into error state right after with rdma_disconnect
> >>>>>>>>>> you are not sure that none of the subset of the invalidations
> >>>>>>>>>> that _were_ posted completed and you get the corresponding MRs
> >>>>>>>>>> in a bogus state...
> >>>>>>>>>
> >>>>>>>>> Moving the QP to error state and then draining the CQs means
> >>>>>>>>> that all LOCAL_INV WRs that managed to get posted will get
> >>>>>>>>> completed or flushed. That's already handled today.
> >>>>>>>>>
> >>>>>>>>> It's the WRs that didn't get posted that I'm worried about
> >>>>>>>>> in this patch.
> >>>>>>>>>
> >>>>>>>>> Are there RDMA consumers in the kernel that use that third
> >>>>>>>>> argument to recover when LOCAL_INV WRs cannot be posted?
> >>>>>>>>
> >>>>>>>> None :)
> >>>>>>>>
> >>>>>>>>>>> I suppose I could reset these MRs instead (that is,
> >>>>>>>>>>> pass them to ib_dereg_mr).
> >>>>>>>>>>
> >>>>>>>>>> Or, just wait for a completion for those that were posted
> >>>>>>>>>> and then all the MRs are in a consistent state.
> >>>>>>>>>
> >>>>>>>>> When a LOCAL_INV completes with IB_WC_SUCCESS, the associated
> >>>>>>>>> MR is in a known state (ie, invalid).
> >>>>>>>>>
> >>>>>>>>> The WRs that flush mean the associated MRs are not in a known
> >>>>>>>>> state. Sometimes the MR state is different than the hardware
> >>>>>>>>> state, for example. Trying to do anything with one of these
> >>>>>>>>> inconsistent MRs results in IB_WC_BIND_MW_ERR until the thing
> >>>>>>>>> is deregistered.
> >>>>>>>>
> >>>>>>>> Correct.
> >>>>>>>>
> >>>>>>>
> >>>>>>> It is legal to invalidate an MR that is not in the valid state.  So
you
> >>>>> don't
> >>>>>>> have to deregister it, you can assume it is valid and post another
LINV
> >>> WR.
> >>>>>>
> >>>>>> I've tried that. Once the MR is inconsistent, even LOCAL_INV
> >>>>>> does not work.
> >>>>>>
> >>>>>
> >>>>> Maybe IB Verbs don't mandate that invalidating an invalid MR must be
> >>> allowed?
> >>>>> (looking at the verbs spec now).
> >>>>
> >>>
> >>> IB Verbs doesn't have specify this requirement.  iW verbs does.  So
> > transport
> >>> independent applications cannot rely on it.  So ib_dereg_mr() seems to be
> > the
> >>> only thing you can do.
> >>>
> >>>> If the MR is truly invalid, then there is no issue, and
> >>>> the second LOCAL_INV completes successfully.
> >>>>
> >>>> The problem is after a flushed LOCAL_INV, the MR state
> >>>> sometimes does not match the hardware state. The MR is
> >>>> neither registered or invalid.
> >>>>
> >>>
> >>> There is a difference, at least with iWARP devices, between the MR state:
> > VALID
> >>> vs INVALID, and if the MR is allocated or not.
> >>>
> >>>> A flushed LOCAL_INV tells you nothing more than that the
> >>>> LOCAL_INV didn't complete. The MR state at that point is
> >>>> unknown.
> >>>>
> >>>
> >>> With respect to iWARP and cxgb4: when you allocate a fastreg MR, HW has an
> >> entry
> >>> for that MR and it is marked "allocated".  The MR record in HW also has a
> > state:
> >>> VALID or INVALID.  While the MR is "allocated" you can post WRs to
> > invalidate it
> >>> which changes the state to INVALID, or fast-register memory which makes it
> >>> VALID.  Regardless of what happens on any given QP, the MR remains
> > "allocated"
> >>> until you call ib_dereg_mr().  So at least for cxgb4, you could in fact
just
> >>> post another LINV to get it back to a known state that allows subsequent
> >>> fast-reg WRs.
> >>>
> >>> Perhaps IB devices don't work this way.
> >>>
> >>> What error did you get when you tried just doing an LINV after a flush?
> >>
> >> With CX-2 and CX-3, after a flushed LOCAL_INV, trying either
> >> a FASTREG or LOCAL_INV on that MR can sometimes complete with
> >> IB_WC_MW_BIND_ERR.
> >
> >
> > I wonder if you post a FASREG+LINV+LINV if you'd get the same failure?  IE
> > invalidate the same rkey twice.  Just as an experiment...
> 
> Once the MR is in this state, FASTREG does not work either.
> All FASTREG and LINV flush with IB_WC_MW_BIND_ERR until
> the MR is deregistered.

Mellanox can probably tell us why. 

I was just wondering if posting a double LINV on a valid working FRMR would fail
with these devices.  But its moot.  As you've concluded, looks like the only
safe was to handle this is to dereg them and reallocate...


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