[PATCH] drm/ttm: make ttm_bo_unpin more defensive

[Thomas Hellström (Intel)]

On 3/16/21 12:06 PM, Daniel Vetter wrote:
> On Tue, Mar 16, 2021 at 11:38:53AM +0100, Thomas Hellström (Intel) wrote:
>> Hi,
>>
>> On 3/16/21 10:27 AM, Daniel Vetter wrote:
>>> On Mon, Mar 15, 2021 at 08:00:30PM +0100, Thomas Hellström (Intel) wrote:
>>>> On 3/15/21 7:47 PM, Christian König wrote:
>>>>> Am 15.03.21 um 18:08 schrieb Thomas Hellström (Intel):
>>>>>> On 3/15/21 11:26 AM, Christian König wrote:
>>>>>>> Am 13.03.21 um 19:29 schrieb Thomas Hellström (Intel):
>>>>>>>> Hi, Christian
>>>>>>>>
>>>>>>>> On 3/12/21 10:38 AM, Christian König wrote:
>>>>>>>>> We seem to have some more driver bugs than thought.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Christian König <christian.koenig at amd.com>
>>>>>>>>> ---
>>>>>>>>>     include/drm/ttm/ttm_bo_api.h | 6 ++++--
>>>>>>>>>     1 file changed, 4 insertions(+), 2 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/include/drm/ttm/ttm_bo_api.h
>>>>>>>>> b/include/drm/ttm/ttm_bo_api.h
>>>>>>>>> index 4fb523dfab32..df9fe596e7c5 100644
>>>>>>>>> --- a/include/drm/ttm/ttm_bo_api.h
>>>>>>>>> +++ b/include/drm/ttm/ttm_bo_api.h
>>>>>>>>> @@ -603,9 +603,11 @@ static inline void
>>>>>>>>> ttm_bo_pin(struct ttm_buffer_object *bo)
>>>>>>>>>     static inline void ttm_bo_unpin(struct ttm_buffer_object *bo)
>>>>>>>>>     {
>>>>>>>>>         dma_resv_assert_held(bo->base.resv);
>>>>>>>>> -    WARN_ON_ONCE(!bo->pin_count);
>>>>>>>>>         WARN_ON_ONCE(!kref_read(&bo->kref));
>>>>>>>>> -    --bo->pin_count;
>>>>>>>>> +    if (bo->pin_count)
>>>>>>>>> +        --bo->pin_count;
>>>>>>>>> +    else
>>>>>>>>> +        WARN_ON_ONCE(true);
>>>>>>>>>     }
>>>>>>>>>       int ttm_mem_evict_first(struct ttm_device *bdev,
>>>>>>>> Since I now have been staring for half a year at the code of
>>>>>>>> the driver that made pinning an art, I have a couple of
>>>>>>>> suggestions here, Could we use an atomic for pin_count,
>>>>>>>> allowing unlocked unpinning but require the lock only for
>>>>>>>> pin_count transition 0->1, (but unlocked for
>>>>>>>> pin_if_already_pinned). In particular I think vmwgfx would
>>>>>>>> benefit from unlocked unpins. Also if the atomic were a
>>>>>>>> refcount_t, that would probably give you the above
>>>>>>>> behaviour?
>>>>>>> Nope, I've considered this as well while moving the pin count into TTM.
>>>>>>>
>>>>>>> The problem is that you not only need the BO reserved for 0->1
>>>>>>> transitions, but also for 1->0 transitions to move the BO on the
>>>>>>> LRU correctly.
>>>>>> Ah, and there is no way to have us know the correct LRU list without
>>>>>> reservation?
>>>>> Not really, there is always the chance that CPU A is reducing the count
>>>>> from 1->0 while CPU B is doing 0->1 and you end up with a LRU status
>>>>> which doesn't match the pin count.
>>>>>
>>>>> We could try to do something like a loop updating the LRU status until
>>>>> it matches the pin count, but the implications of that are usually
>>>>> really nasty.
>>>>>
>>>> OK, yeah I was more thinking along the lines of protecting the LRU status
>>>> with the global lru lock and unpin would then be
>>>>
>>>> if (refcount_dec_and_lock(&bo->pin_count, &ttm_glob.lru_lock)) {
>>>>       add_to_relevant_lrus(bo, bo->lru_status);
>>>>       spin_unlock(&ttm_glob.lru_lock);
>>>> }
>>>>
>>>> But looking at ttm_bo_move_to_lru_tail() I realize that's not really trivial
>>>> anymore. I hope it's doable at some point though.
>>>>
>>>> But meanwhile, perhaps TTM needs to accept and pave over that drivers are in
>>>> fact destroying pinned buffers?
>>> Do we have more trouble than the very fancy tricks vmwgfx does? If so I
>>> think we could do a small helper that like ttm_dont_check_unpin() to shut
>>> it up. Since vmwgfx drivers tend to not be loaded with any other drivers
>>> that shouldn't reduce any test coverage. And allows vmwgfx folks to figure
>>> out what to do - maybe you do need your own in-house pin/unpin for these
>>> special bo?
>>>
>>> I did try to parse your reply in the other thread, and tbh I didn't grock
>>> it.
>> Not sure if you mean the description was unclear or if you thought it was a
>> bad idea, but in case the former, what I mean is
> My unclarity is on what you explained in the vmwgfx thread about how
> vmwgfx uses its pin/unpin and why. That was full of vmwgfx concepts I
> don't know about. This here looks reasonably clear to me, but it does have
> the race Christian sees I think.

Hmm, OK, I thought transition 0->1 under the LRU lock would have 
resolved that...

>> static void ttm_bo_pin(struct ttm_buffer_object *bo)
>> {
>>
>> dma_resv_assert_held()                        // No surprises if an evictor
>> determined that this object is not pinned.
>>
>>      if (!refcount_inc_not_zero(&bo->pin_count)) { // Could be made
>> ttm_bo_pin_if_pinned() and exported if there are users
>>          spin_lock(&ttm_glob.lru_lock);            // Don't race with unpin
>> 1->0
>>          if (refcount_read(&bo->pin_count) == 0 && bo->lru)
>>              ttm_bo_del_from_lru(bo);
>>          refcount_inc(&bo->pin_count);
>>          spin_unlock(&ttm_glob.lru_lock);
>>      }
>> }
>>
>> static void ttm_bo_unpin(struct ttm_buffer_object *bo)
>> {
>>      if (refcount_dec_and_lock(&bo->pin_count, &ttm_glob.lru_lock)) {
>>          ttm_bo_move_to_lru_tail(bo, bo->lru_mem_type, bo->lru_prio,
>>                      NULL);
>>          spin_unlock(&ttm_glob.lru_lock);
>>      }
>> }
>>
>> In addition, bo->lru_mem_type and bo->lru_prio would need to be protected by
>> the lru lock, and updated together with the LRU list position, which would
>> be the extra complexity in fastpaths. Wouldn't that resolve the pin - lru
>> inconsistency?
>>
>> But yeah if vmwgfx is the only driver hitting trouble because of this, then
>> I agree let's leave it for when / if it becomes needed. Having had that
>> requirement in the Intel driver would have complicated the dma_resv work
>> quite a lot.
> Yeah I know i915 does a lot of unpin in interesting places, and that's
> part of why I'm worried. I've seen some bugfixes fly by that dropped
> dma_resv_lock around unpin to fix really scary looking stalls
> (rcu_synchronize vs other stuff and lockdep did not catch it). And once I
> see that kind of stuff I'm heavily leaning towards simpler locking we can
> grasp, just to be able to stay on top of the bugs. Because the bugfix did
> not come with any clear explanation for why not taking dma_resv_lock
> actually helps, or any other clear and in-depth locking analysis.
>
> I think we also still have some temporary pin/unpin flying about, but
> maybe those are all gone now.

No, unfortunately they aren't and yes, I now see your point. And yes, if 
we can reduce the pinning to only display kinds of things this wouldn't 
really matter. But if not, I'm afraid the alternative is that drivers 
will have to start resorting to assuming perhaps incorrectly that an 
object is isolated (only a single user), and that therefore tryreserve 
for unpinning will always succeed, which is fragile at best. (This is 
what vmwgfx does for the pagetables). And pinning pagetables in that 
case makes sense since it reduces the number of buffers to validate and 
fence by 50%...

/Thomas