[PATCH 2/2] drm/vmwgfx: Make sure unpinning handles reservations

[Zack Rusin]

On 4/9/21 3:40 AM, Daniel Vetter wrote:
> On Thu, Apr 08, 2021 at 01:22:45PM -0400, Zack Rusin wrote:
>> Quite often it's a little hard to tell if reservations are already held
>> in code paths that unpin bo's. While our pinning/unpinning code should
>> be more explicit that requires a substential amount of work so instead
>> we can avoid the issues by making sure we try to reserve before unpinning.
>> Because we unpin those bo's only on destruction/error paths just that check
>> tells us if we're already reserved or not and allows to cleanly unpin.
>>
>> Reviewed-by: Martin Krastev <krastevm at vmware.com>
>> Reviewed-by: Roland Scheidegger <sroland at vmware.com>
>> Fixes: d1a73c641afd ("drm/vmwgfx: Make sure we unpin no longer needed buffers")
>> Cc: dri-devel at lists.freedesktop.org
>> Signed-off-by: Zack Rusin <zackr at vmware.com>
>> ---
>>   drivers/gpu/drm/vmwgfx/vmwgfx_drv.h | 17 ++++++++++++++++-
>>   drivers/gpu/drm/vmwgfx/vmwgfx_mob.c |  8 ++++----
>>   2 files changed, 20 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h
>> index 8087a9013455..03bef9c17e56 100644
>> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h
>> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h
>> @@ -1517,6 +1517,21 @@ static inline struct vmw_surface *vmw_surface_reference(struct vmw_surface *srf)
>>   	return srf;
>>   }
>>   
>> +/*
>> + * vmw_bo_unpin_safe - currently pinning requires a reservation to be held
>> + * but sometimes it's hard to tell if we're in a callback whose parent
>> + * is already holding a reservation, to avoid deadloacks we have to try
>> + * to get a reservation explicitly to also try to avoid messing up the
>> + * internal ttm lru bo list
>> + */
>> +static inline void vmw_bo_unpin_safe(struct ttm_buffer_object *bo)
>> +{
>> +	bool locked = dma_resv_trylock(bo->base.resv);
>> +	ttm_bo_unpin(bo);
>> +	if (locked)
>> +		dma_resv_unlock(bo->base.resv);
>> +}
>> +
>>   static inline void vmw_bo_unreference(struct vmw_buffer_object **buf)
>>   {
>>   	struct vmw_buffer_object *tmp_buf = *buf;
>> @@ -1524,7 +1539,7 @@ static inline void vmw_bo_unreference(struct vmw_buffer_object **buf)
>>   	*buf = NULL;
>>   	if (tmp_buf != NULL) {
>>   		if (tmp_buf->base.pin_count > 0)
>> -			ttm_bo_unpin(&tmp_buf->base);
>> +			vmw_bo_unpin_safe(&tmp_buf->base);
> 
> So in the unreference callback I understand it might be tricky and you
> need this, but do all the others below really don't know whether the bo is
> locked or not?

TBH, I just liked having all those paths going through the same 
functions. I agree that it wasn't really correct or particularly graceful.

> Also _trylock is a bit much yolo locking here, I'd minimally put a comment
> there that we don't actually care about races, it's just to shut up ttm
> locking checks. Whether that's true or not is another question I think.
> 
> And if it's just this case here, maybe inline the trylock, and for the
> others do a vmw_bo_unpin_unlocked which unconditionally grabs the lock?

Fair enough, I think that's a good suggestion, so I went ahead and did 
just that.

z