[PATCH v3 3/4] drm/ttm: convert to unified vma offset manager

[David Herrmann]

Hi

On Thu, Jul 18, 2013 at 1:24 PM, Thomas Hellstrom <thellstrom at vmware.com> wrote:
> On 07/18/2013 01:07 PM, David Herrmann wrote:
>>
>> Hi
>>
>> On Thu, Jul 18, 2013 at 10:53 AM, Thomas Hellstrom
>> <thellstrom at vmware.com> wrote:
>>>
>>> A quick look, but not a full review:
>>>
>>> Looks mostly good, but it looks like the TTM vm lock isn't needed at all
>>> anymore (provided the vma offset manager is properly protected), since
>>> kref_get_unless_zero() is used when a reference after lookup is taken.
>>> (please see the kref_get_unless_zero documentation examples). When
>>> drm_vma_offset_remove() is called, the kref value is unconditionally
>>> zero,
>>> and that should block any successful lookup.
>>
>> If we drop vm_lock without modifying TTM, this will not work. Even
>> kref_get_unless_zero() needs some guarantee that the object is still
>> valid. Assume this scenario:
>>
>> drm_vma_offset_lookup() returns a valid node, however, before we call
>> kref_get_*(), another thread destroys the last reference to the bo so
>> it gets kfree()d. kref_get_unless_zero() will thus reference memory
>> which can be _anything_ and is not guarantee to stay 0.
>> (Documentation/kref.txt mentions this, too, and I guess it was you who
>> wrote that).
>>
>> I cannot see any locking around the mmap call that could guarantee
>> this except vm_lock.
>
>
> You're right. My apologies. Parental leave has taken its toll.
>
>
>>
>>> Otherwise, if the vma offset manager always needs external locking to
>>> make
>>> lookup + referencing atomic, then perhaps locking should be completely
>>> left to the caller?
>>
>> I would actually prefer to keep it in the VMA manager. It allows some
>> fast-paths which otherwise would need special semantics for the caller
>> (for instance see the access-management patches which are based on
>> this patchset or the reduced locking during setup/release). We'd also
>> require the caller to use rwsems for good performance, which is not
>> the case for gem.
>>
>> So how about Daniel's proposal to add an _unlocked() version and
>> provide _lock_lookup() and _unlock_lookup() helpers which just wrap
>> the read_lock() and read_unlock?
>>
>> Thanks!
>> David
>
>
> I think that if there are good reasons to keep locking internal, I'm fine
> with that, (And also, of course, with
> Daniel's proposal). Currently the add / remove / lookup paths are mostly
> used by TTM during object creation and
> destruction.
>
> However, if the lookup path is ever used by pread / pwrite, that situation
> might change and we would then like to
> minimize the locking.

I tried to keep the change as minimal as I could. Follow-up patches
are welcome. I just thought pushing the lock into drm_vma_* would
simplify things. If there are benchmarks that prove me wrong, I'll
gladly spend some time optimizing that.

Apart from this, I'd like to see someone ack the
ttm_buffer_object_transfer() changes. I am not very confident with
that. Everything else should be trivial.

Thanks
David