On Fri, 2021-04-23 at 13:50 +0200, Halil Pasic wrote: > On Thu, 22 Apr 2021 16:49:21 -0400 > Eric Farman <farman@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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
