[PATCH 1/5] dma-buf: remove fallback for !CONFIG_DMA_SHARED_BUFFER

[Thomas Hellstrom]

On 09/28/2012 09:42 PM, Thomas Hellstrom wrote:
> On 09/28/2012 04:14 PM, Maarten Lankhorst wrote:
>> Hey,
>>
>> Op 28-09-12 14:41, Maarten Lankhorst schreef:
>>> Documentation says that code requiring dma-buf should add it to
>>> select, so inline fallbacks are not going to be used. A link error
>>> will make it obvious what went wrong, instead of silently doing
>>> nothing at runtime.
>>>
>>
>>
>> The whole patch series is in my tree, I use stg so things might
>> move around, do not use for merging currently:
>>
>> http://cgit.freedesktop.org/~mlankhorst/linux/log/?h=v10-wip
>>
>> It contains everything in here plus the patches for ttm to make
>> it work, I use a old snapshot of drm-next + merge of nouveau as
>> base. Description of what the parts do:
>>
>> Series to fix small api issues when moving over:
>>
>> drm/ttm: Remove cpu_writers related code
>> drm/ttm: Add ttm_bo_is_reserved function
>> drm/radeon: Use ttm_bo_is_reserved
>> drm/vmwgfx: use ttm_bo_is_reserved
>>
>> drm/vmwgfx: remove use of fence_obj_args
>> drm/ttm: remove sync_obj_arg
>> drm/ttm: remove sync_obj_arg from ttm_bo_move_accel_cleanup
>> drm/ttm: remove sync_arg entirely
>>
>> drm/nouveau: unpin buffers before releasing to prevent lockdep warnings
>> drm/nouveau: add reservation to nouveau_bo_vma_del
>> drm/nouveau: add reservation to nouveau_gem_ioctl_cpu_prep
>>
>> Hey great, now we only have one user left for fence waiting before 
>> reserving,
>> lets fix that and remove fence lock:
>> ttm_bo_cleanup_refs_or_queue and ttm_bo_cleanup_refs have to reserve 
>> before
>> waiting, lets do it in the squash commit so we don't have to throw 
>> lock order
>> around everywhere:
>>
>> drm/ttm: remove fence_lock
>>
>> -- Up to this point should be mergeable now
>>
>> Then we start working on lru_lock removal slightly, this means the lru
>> list no longer is empty but can contain only reserved buffers:
>>
>> drm/ttm: do not check if list is empty in ttm_bo_force_list_clean
>> drm/ttm: move reservations for ttm_bo_cleanup_refs
>>
>> -- Still mergeable up to this point, just fixes
>>
>> Patch series from this email:
>> dma-buf: remove fallback for !CONFIG_DMA_SHARED_BUFFER
>> fence: dma-buf cross-device synchronization (v9)
>> seqno-fence: Hardware dma-buf implementation of fencing (v3)
>> reservation: cross-device reservation support
>> reservation: Add lockdep annotation and selftests
>>
>> Now hook it up to drm/ttm in a few steps:
>> usage around reservations:
>> drm/ttm: make ttm reservation calls behave like reservation calls
>> drm/ttm: use dma_reservation api
>> dma-buf: use reservations
>> drm/ttm: allow drivers to pass custom dma_reservation_objects for a bo
>>
>> then kill off the lru lock around reservation:
>> drm/ttm: remove lru_lock around ttm_bo_reserve
>> drm/ttm: simplify ttm_eu_*
>>
>> The lru_lock removal patch removes the lock around lru_lock around the
>> reservation, this will break the assumption that items on the lru list
>> and swap list can always be reserved, and this gets patched up too.
>> Is there any part in ttm you disagree with? I believe that this
>> is all mergeable, the lru_lock removal patch could be moved to before
>> the reservation parts, this might make merging easier, but I don't
>> think there is any ttm part of the series that are wrong on a conceptual
>> level.
>>
>> ~Maarten
>>
> ....From another email
>
>>> As previously discussed, I'm unfortunately not prepared to accept 
>>> removal of the reserve-lru atomicity
>>>   into the TTM code at this point.
>>> The current code is based on this assumption and removing it will 
>>> end up with
>>> efficiencies, breaking the delayed delete code and probably a 
>>> locking nightmare when trying to write
>>> new TTM code.
>> The lru lock removal patch fixed the delayed delete code, it really 
>> is not different from the current
>> situation. In fact it is more clear without the guarantee what 
>> various parts are trying to protect.
>>
>> Nothing prevents you from holding the lru_lock while trylocking,
> [1]
> While this would not cause any deadlocks, Any decent lockdep code 
> would establish lru->reserve as the locking
> order once a lru- reserve trylock succeeds, but the locking order is 
> really reserve->lru for obvious reasons, which
> means we will get a lot of lockdep errors? Yes, there are a two 
> reversals like these already in the TTM code, and I'm
> not very proud of them.
>
> leaving that guarantee intact for that part. Can you really just review
> the patch and tell me where it breaks and/or makes the code unreadable?
>
> OK. Now I'm looking at
> http://cgit.freedesktop.org/~mlankhorst/linux/tree/drivers/gpu/drm/ttm/ttm_bo.c?h=v10-wip&id=1436e2e64697c744d6e186618448e1e6354846bb 
>
>
> And let's start with a function that's seen some change, 
> ttm_mem_evict_first:
>
> *) Line 715: You're traversing a list using list_for_each() calling a 
> function that may remove the list entr||||y
> *) Line 722: You're unlocking the lock protecting the list in the 
> middle of list traversal
> *) Line 507: WARN_ON_ONCE in a code path quite likely to get called?
> *) Line 512: sleep while atomic
> *) Line 729: Forgot to unreserve
> *) Line 730: Forgot to lock lru
> *) Line 735: Now you're restarting with the first item on the LRU 
> list. Why the loop at line 715?
> *) Line 740: Deadlocking reserve
> *) Line 757: Calling TTM Bo evict, but there might have been another 
> process already evicting the buffer while
> you released the lru_lock in line 739, before reserving the buffer.

Actually, Lines 715, 722, 730 and 735 are OK, sorry about that, but the 
others should be valid.
/Thomas