On Mon, May 16, 2022 at 10:00:07AM -0400, Demi Marie Obenour wrote: > On Mon, May 16, 2022 at 08:48:17AM +0200, Juergen Gross wrote: > > On 14.05.22 17:55, Demi Marie Obenour wrote: > > > In https://github.com/QubesOS/qubes-issues/issues/7481, a user reported > > > that Xorg locked up when resizing a VM window. While I do not have the > > > same hardware the user does and thus cannot reproduce the bug, the stack > > > trace seems to indicate a deadlock between xen_gntdev and i915. It > > > appears that gnttab_unmap_refs_sync() is waiting for i915 to free the > > > pages, while i915 is waiting for the MMU notifier that called > > > gnttab_unmap_refs_sync() to return. Result: deadlock. > > > > > > The problem appears to be that a mapped grant in PV mode will stay in > > > the “invalidating” state until it is freed. While MMU notifiers are > > > allowed to sleep, it appears that they cannot wait for the page to be > > > freed, as is happening here. That said, I am not very familiar with > > > this code, so my diagnosis might be incorrect. > > > > All I can say for now is that your patch seems to be introducing a use after > > free issue, as the parameters of the delayed work might get freed now before > > the delayed work is being executed. > > I figured it was wrong, not least because I don’t think it compiles > (invalid use of void value). That said, the current behavior is quite > suspicious to me. For one, it appears that munmap() on a grant in a PV > domain will not return until nobody else is using the page. This is not > what I would expect, and I can easily imagine it causing deadlocks in > userspace. Instead, I would expect for gntdev to automatically release > the grant when the reference count hits zero. This would also allow for > the same grant to be mapped in multiple processes, and might even unlock > DMA-BUF support. > > > I don't know why this is happening only with rather recent kernels, as the > > last gntdev changes in this area have been made in kernel 4.13. > > > > I'd suggest to look at i915, as quite some work has happened in the code > > visible in your stack backtraces rather recently. Maybe it would be possible > > to free the pages in i915 before calling the MMU notifier? > > While I agree that the actual problem is almost certainly in i915, the > gntdev code does appear rather fragile. Since so few people use i915 + > Xen, problems with the combination generally don’t show up until some > Qubes user makes a bug report, which isn’t great. It would be better if > Xen didn’t introduce requirements on other kernel code that did not hold > when not running on Xen. > > In this case, if it is actually an invariant that one must not call MMU > notifiers for pages that are still in use, it would be better if this > was caught by a WARN_ON() or BUG_ON() in the core memory management > code. That would have found the bug instantly and deterministically on > all platforms, whereas the current failure is nondeterministic and only > happens under Xen. > > I also wonder if this is a bug in the core MMU notifier infrastructure. > My reading of the mmu_interval_notifier_remove() documentation is that > it should only wait for the specific notifier being removed to finish, > not for all notifiers to finish. Adding the memory management > maintainers. Also adding the kernel regression tracker. #regzbot introduced v5.16..v5.17.4 -- Sincerely, Demi Marie Obenour (she/her/hers) Invisible Things Lab
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