On Thu, 2021-05-13 at 03:05 +0200, Halil Pasic wrote: > On Tue, 11 May 2021 21:56:28 +0200 > Eric Farman <farman@xxxxxxxxxxxxx> wrote: > > > Hi Conny, Matt, Halil, > > > > Here's one (last?) update to my proposal for handling the collision > > between interrupts for START SUBCHANNEL and HALT/CLEAR SUBCHANNEL. > > > > Only change here is to include Conny's suggestions on patch 3. > > > > Thanks, > > I believe these changes are beneficial, although I don't understand > everything about them. In that sense I'm happy with the these getting > merged. > > Let me also spend some words answering the unasked question, what I'm > not understanding about these. > > Not understanding how the problem stated in the cover letter of v4 is > actually resolved is certainly the most important one. Per our phone call last week, one of Conny's suggestions from that particular version was related to vfio_ccw_sch_io_todo() and was giving me some difficulties. We all agreed that I should send what I had, and leave the other corner case(s) to be addressed later along with the broader serialization topic throughout the driver. That is still my intention, but I suspect that's where you are going here... (I realize I said "last?" at the top here. Poor decision on my part.) > Let me cite > the relevant part of it (your cover letter already contains a link to > the full version). > > """ > > 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. > """ > > The last sentence clearly states "may release memory that is actively > used by ... the device", and I understood it refers to the invocation > of cp_free() from vfio_ccw_sch_io_todo(). Patch 3 of this series does > not change the conditions under which cp_free() is called. Correct. > > Looking at the cited diagram, since patch 3 changes things in > vfio_ccw_sch_io_todo() it probably ain't affecting steps 1-3 and > I understood the description so that bad free happens in step 4. You are correct that patch 3 touches vfio_ccw_sch_io_todo(), but it is not addressing the possibility of a bad free described in the old cover letter. The commit message for patch 3 describes pretty clearly the scenario in question. > > My guess is that your change from patch 3 somehow via the fsm > prevents > the SSCH on CPU 2 (using the diagram) from being executed if it > actually > happens to be after vfio_ccw_sch_io_todo(). That's an incorrect guess. The code in vfio_ccw_sch_io_todo() today says "If another CPU is building an I/O (FSM is CP_PROCESSING), or there is no CPU building an I/O (FSM is IDLE), then skip the cp_free() call." The change in patch 3 says that in that situation, it should also not adjust the FSM state because the interrupt being handled on CPU1 was unrelated (maybe it was for a HALT/CLEAR, maybe it was an unsolicited interrupt). The SSCH on CPU2 will still go on as expected. > And patch 1 is supposed to > prevent the SSCH on CPU2 from being executed in the depicted case > because > if there is a cp to free, then we would bail out form if we see it > while processing the new IO request. Not really. It's the FSM's job to prevent a second SSCH, and route to fsm_io_retry() or fsm_io_busy() as appropriate. But the scenario described by patch 3 in this series would leave the cp initialized, while also resetting the FSM back to IDLE. As such, the FSM was free to allow another SSCH in, which would then re-initialize the cp and orphan the existing (active) cp resources. With the application of patch 3, that concern isn't present, so the change in patch 1 is really a NOP. But it allows for consistency in how the cp_*() functions are working, and a safety valve should this situation show up another way. (We'll get trace data that says cp_init() bailed out, rather than going on as if nothing were wrong.) > > In any case, I don't want to hold this up any further. > Thanks for that. You are correct that there's still a potential issue here, in the handoff between fsm_irq() and vfio_ccw_sch_io_todo(), and another fsm_io_request() that would arrive between those points. But it's not anything that we haven't already discussed, and will hopefully begin discussing in the next couple of weeks. Thanks, Eric > Regards, > Halil
