[Intel-gfx] [PATCH v3 3/3] drm/i915: Use a task to cancel the userptr on invalidate_range

[Chris Wilson]

On Wed, Sep 09, 2015 at 04:20:08PM +0100, Tvrtko Ursulin wrote:
> 
> On 09/09/2015 04:08 PM, Chris Wilson wrote:
> >On Wed, Sep 09, 2015 at 03:45:40PM +0100, Tvrtko Ursulin wrote:
> >>On 08/10/2015 09:51 AM, Chris Wilson wrote:
> >>>Whilst discussing possible ways to trigger an invalidate_range on a
> >>>userptr with an aliased GGTT mmapping (and so cause a struct_mutex
> >>>deadlock), the conclusion is that we can, and we must, prevent any
> >>>possible deadlock by avoiding taking the mutex at all during
> >>>invalidate_range. This has numerous advantages all of which stem from
> >>>avoid the sleeping function from inside the unknown context. In
> >>>particular, it simplifies the invalidate_range because we no longer
> >>>have to juggle the spinlock/mutex and can just hold the spinlock
> >>>for the entire walk. To compensate, we have to make get_pages a bit more
> >>>complicated in order to serialise with a pending cancel_userptr worker.
> >>>As we hold the struct_mutex, we have no choice but to return EAGAIN and
> >>>hope that the worker is then flushed before we retry after reacquiring
> >>>the struct_mutex.
> >>>
> >>>The important caveat is that the invalidate_range itself is no longer
> >>>synchronous. There exists a small but definite period in time in which
> >>>the old PTE's page remain accessible via the GPU. Note however that the
> >>>physical pages themselves are not invalidated by the mmu_notifier, just
> >>>the CPU view of the address space. The impact should be limited to a
> >>>delay in pages being flushed, rather than a possibility of writing to
> >>>the wrong pages. The only race condition that this worsens is remapping
> >>>an userptr active on the GPU where fresh work may still reference the
> >>>old pages due to struct_mutex contention. Given that userspace is racing
> >>>with the GPU, it is fair to say that the results are undefined.
> >>>
> >>>v2: Only queue (and importantly only take one refcnt) the worker once.
> >>
> >>This one I looked at at the time of previous posting and it looked
> >>fine, minus one wrong line of thinking of mine. On a brief look it
> >>still looks good, so:
> >>
> >>Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >>
> >>I assume Michał has run all these through the relevant test cases?
> >>
> >>Slightly related, I now worry about the WARN_ONs in
> >>__cancel_userptr__worker since they look to be triggerable by
> >>malicious userspace which is not good.
> >
> >They could always be I thought, if you could somehow pin the userptr
> >into a hardware register and then unmap the vma. That is a scary thought
> >and one I would like a WARN for. That should be the only way, and I shudder
> >at the prospect of working out who to send the SIGBUS to.
> 
> Is it not enough to submit work to the GPU and while it is running
> engineer a lot of signals and munmap?

No, we block signals inside the worker, which should reduce it down to
EINVAL/EBUSY or EIO from unbind (and a subsequent WARN from put).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre