Re: [RFC PATCH v4 0/4] vfio-ccw: Fix interrupt handling for HALT/CLEAR

[Eric Farman <farman@linux.ibm.com>]

On Fri, 2021-04-23 at 13:50 +0200, Halil Pasic wrote:
> On Thu, 22 Apr 2021 16:49:21 -0400
> Eric Farman <farman@linux.ibm.com> wrote:
> 
> > On Thu, 2021-04-22 at 02:52 +0200, Halil Pasic wrote:
> > > On Tue, 13 Apr 2021 20:24:06 +0200
> > > Eric Farman <farman@linux.ibm.com> wrote:
> > >   
> > > > Hi Conny, Halil,
> > > > 
> > > > Let's restart our discussion about the collision between
> > > > interrupts
> > > > for
> > > > START SUBCHANNEL and HALT/CLEAR SUBCHANNEL. It's been a quarter
> > > > million
> > > > minutes (give or take), so here is the problematic scenario
> > > > again:
> > > > 
> > > > 	CPU 1			CPU 2
> > > >  1	CLEAR SUBCHANNEL
> > > >  2	fsm_irq()
> > > >  3				START SUBCHANNEL
> > > >  4	vfio_ccw_sch_io_todo()
> > > >  5				fsm_irq()
> > > >  6				vfio_ccw_sch_io_todo()
> > > > 
> > > > From the channel subsystem's point of view the CLEAR SUBCHANNEL
> > > > (step 1)
> > > > is complete once step 2 is called, as the Interrupt Response
> > > > Block
> > > > (IRB)
> > > > has been presented and the TEST SUBCHANNEL was driven by the
> > > > cio
> > > > layer.
> > > > Thus, the START SUBCHANNEL (step 3) is submitted [1] and gets a
> > > > cc=0 to
> > > > indicate the I/O was accepted. However, step 2 stacks the bulk
> > > > of
> > > > the
> > > > actual work onto a workqueue for when the subchannel lock is
> > > > NOT
> > > > held,
> > > > and is unqueued at step 4. That code misidentifies the data in
> > > > the
> > > > IRB
> > > > as being associated with the newly active I/O, and may release
> > > > memory
> > > > that is actively in use by the channel subsystem and/or device.
> > > > Eww.
> > > > 
> > > > In this version...
> > > > 
> > > > Patch 1 and 2 are defensive checks. Patch 2 was part of v3 [2],
> > > > but
> > > > I
> > > > would love a better option here to guard between steps 2 and 4.
> > > > 
> > > > Patch 3 is a subset of the removal of the CP_PENDING FSM state
> > > > in
> > > > v3.
> > > > I've obviously gone away from this idea, but I thought this
> > > > piece
> > > > is
> > > > still valuable.
> > > > 
> > > > Patch 4 collapses the code on the interrupt path so that
> > > > changes to
> > > > the FSM state and the channel_program struct are handled at the
> > > > same
> > > > point, rather than separated by a mutex boundary. Because of
> > > > the
> > > > possibility of a START and HALT/CLEAR running concurrently, it
> > > > does
> > > > not make sense to split them here.
> > > > 
> > > > With the above patches, maybe it then makes sense to hold the
> > > > io_mutex
> > > > across the entirety of vfio_ccw_sch_io_todo(). But I'm not
> > > > completely
> > > > sure that would be acceptable.
> > > > 
> > > > So... Thoughts?  
> > > 
> > > I believe we should address  
> > 
> > Who is the "we" here?
> > 
> 
> The people that are responsible for vfio-ccw. 
> 
> > >  the concurrency, encapsulation and layering
> > > issues in the subchannel/ccw pass-through code (vfio-ccw) by
> > > taking a
> > > holistic approach as soon as possible.
> > > 
> > > I find the current state of art very hard to reason about, and
> > > that
> > > adversely  affects my ability to reason about attempts at partial
> > > improvements.
> > > 
> > > I understand that such a holistic approach needs a lot of work,
> > > and
> > > we
> > > may have to stop some bleeding first. In the stop the bleeding
> > > phase
> > > we
> > > can take a pragmatic approach and accept changes that empirically
> > > seem to
> > > work towards stopping the bleeding. I.e. if your tests say it's
> > > better,
> > > I'm willing to accept that it is better.  
> > 
> > So much bleeding!
> > 
> > RE: my tests... I have only been seeing the described problem in
> > pathological tests, and this series lets those tests run without
> > issue.
> > 
> 
> Good to know.
> 
> > > I have to admit, I don't understand how synchronization is done
> > > in
> > > the
> > > vfio-ccw kernel module (in the sense of avoiding data races).
> > > 
> > > Regarding your patches, I have to admit, I have a hard time
> > > figuring
> > > out
> > > which one of these (or what combination of them) is supposed to
> > > solve
> > > the problem you described above. If I had to guess, I would guess
> > > it
> > > is
> > > either patch 4, because it has a similar scenario diagram in the
> > > commit message like the one in the problem statement. Is my guess
> > > right?  
> > 
> > Sort of. It is true that Patch 4 is the last piece of the puzzle,
> > and
> > the diagram is included in that commit message so it is kept with
> > the
> > change, instead of being lost with the cover letter.
> > 
> > As I said in the cover letter, "Patch 1 and 2 are defensive checks"
> > which are simply included to provide a more robust solution. You
> > could
> > argue that Patch 3 should be held out separately, but as it came
> > from
> > the previous version of this series it made sense to include here.
> > 
> 
> Does that mean we need patches 1, 2 and 4 to fix the issue or is just
> 4 sufficient?

Based on everything I understand, I would not feel comfortable with
only 4.

If you look at the commit message for patch 1, I do explain why its
absence is not exposing any serialization problems today. But as it is
part of the "CCW translation API" (documented in
Documentation/s390/vfio-ccw.rst) I feel it is important to include.

Something needs to be done in the transition described by Patch 2, even
though it still has problems in its current form.

> 
> > > If it is right I don't quite understand the mechanics of the fix,
> > > because what the patch seems to do is changing the content of
> > > step 4
> > > in
> > > the above diagram. And I don't see how is change that code
> > > so that it does not "misidentifies the data in the IRB as being
> > > associated with the newly active I/O".   
> > 
> > Consider that the cp_update_scsw() and cp_free() routines that get
> > called here are looking at the cp->initialized flag to determine
> > whether to perform any work. For a system that is otherwise idle,
> > the
> > cp->initialized flag will be false when processing an IRB related
> > to a
> > CSCH, meaning the bulk of this routine will be a NOP.
> > 
> > In the failing scenario, as I describe in the commit message for
> > patch
> > 4, we could be processing an interrupt that is unaffiliated with
> > the CP
> > that was (or is being) built. It need not even be a solicited
> > interrupt; it just happened that the CSCH interrupt is what got me
> > looking at this path. The whole situation boils down to the FSM
> > state
> > and cp->initialized flag being out of sync from one another after
> > coming through this function.
> > 
> 
> Thanks for the explanation. Since you are about to send out a new
> verison which I understand won't be just about cosmetic fixes, I
> won't
> invest any more in understanding this one. But I hope this will help
> me
> understand that one. 
> 
> > > Moreover patch 4 seems to rely on
> > > private->state which, AFAIR is still used in a racy fashion.
> > > 
> > > But if strong empirical evidence shows that it performs better
> > > (stops
> > > the bleeding), I think we can go ahead with it.  
> > 
> > Again with the bleeding. Is there a Doctor in the house? :)
> > 
> 
> Sorry if I expressed myself comically. Was not my intention. I'm
> puzzled.

No need to be sorry, and I didn't mean to offend you. The repeated use
of such a dramatic phrase struck me as humorous, because it conveyed a
much more prevalant problem than the one being fixed.

> 
> Is in your opinion the vfio-ccw kernel module data race free with
> this
> series applied?

I have no further concerns.

Eric

> 
> Regards,
> Halil