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