From: Cornelia Huck <cohuck@redhat.com>
To: Eric Farman <farman@linux.ibm.com>
Cc: Halil Pasic <pasic@linux.ibm.com>,
Matthew Rosato <mjrosato@linux.ibm.com>,
Jared Rossi <jrossi@linux.ibm.com>,
linux-s390@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [RFC PATCH v4 0/4] vfio-ccw: Fix interrupt handling for HALT/CLEAR
Date: Fri, 23 Apr 2021 13:06:16 +0200 [thread overview]
Message-ID: <20210423130616.6dcbf4e4.cohuck@redhat.com> (raw)
In-Reply-To: <1eb9cbdfe43a42a62f6afb0315bb1e3a103dac9a.camel@linux.ibm.com>
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 concurrency, encapsulation and layering
> > issues in the subchannel/ccw pass-through code (vfio-ccw) by taking a
> > holistic approach as soon as possible.
Let me also ask: what is "holistic"? If that's a complete rewrite, I
definitely don't have the capacity for that; if others want to take
over the code, feel free.
> >
> > 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.
FWIW, I haven't been able to reproduce the problem myself, and I don't
remember seeing other reports. It's still a problem, and if we can get
rid of the issue, good. The reasoning about what is happening makes
sense to me.
>
> >
> > 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.
>
> >
> > 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.
Nod, that's also my understanding.
>
> > 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? :)
No idea, seen any blue boxes around? :)
next prev parent reply other threads:[~2021-04-23 11:06 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-13 18:24 [RFC PATCH v4 0/4] vfio-ccw: Fix interrupt handling for HALT/CLEAR Eric Farman
2021-04-13 18:24 ` [RFC PATCH v4 1/4] vfio-ccw: Check initialized flag in cp_init() Eric Farman
2021-04-14 16:30 ` Cornelia Huck
2021-04-13 18:24 ` [RFC PATCH v4 2/4] vfio-ccw: Check workqueue before doing START Eric Farman
2021-04-15 10:51 ` Cornelia Huck
2021-04-15 13:48 ` Eric Farman
2021-04-15 16:19 ` Cornelia Huck
2021-04-15 18:42 ` Eric Farman
2021-04-16 14:41 ` Cornelia Huck
2021-04-13 18:24 ` [RFC PATCH v4 3/4] vfio-ccw: Reset FSM state to IDLE inside FSM Eric Farman
2021-04-15 10:54 ` Cornelia Huck
2021-04-13 18:24 ` [RFC PATCH v4 4/4] vfio-ccw: Reset FSM state to IDLE before io_mutex Eric Farman
2021-04-21 10:25 ` Cornelia Huck
2021-04-21 12:58 ` Eric Farman
2021-04-22 16:16 ` Eric Farman
2021-04-22 0:52 ` [RFC PATCH v4 0/4] vfio-ccw: Fix interrupt handling for HALT/CLEAR Halil Pasic
2021-04-22 20:49 ` Eric Farman
2021-04-23 11:06 ` Cornelia Huck [this message]
2021-04-23 13:23 ` Halil Pasic
2021-04-23 13:28 ` Niklas Schnelle
2021-04-23 15:53 ` Eric Farman
2021-04-23 11:50 ` Halil Pasic
2021-04-23 15:53 ` Eric Farman
2021-04-23 17:08 ` Halil Pasic
2021-04-23 19:07 ` Eric Farman
2021-04-24 0:18 ` Halil Pasic
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