Re: Hang in 5.17.4+ that appears to be due to Xen

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

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.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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