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

[Tvrtko Ursulin]

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?

Regards,

Tvrtko