Date:	Fri, 28 Sep 2012 21:42:34 +0200
From:	Thomas Hellstrom <thellstrom@...are.com>
To:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
CC:	jakob@...are.com, dri-devel@...ts.freedesktop.org,
	sumit.semwal@...aro.org, linaro-mm-sig@...ts.linaro.org,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] dma-buf: remove fallback for !CONFIG_DMA_SHARED_BUFFER

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.

And this is even before it starts to get interesting, like how you 
guarantee that when you release a buffer from
the delayed delete list, you're the only process having a reference?

Now, it's probably possible to achieve what you're trying to do, if we 
accept the lock reversal in
[1], but since I have newborn twins and I have about one hour of spare 
time a week with I now spent on this
review and I guess there are countless more hours before this can work. 
(These code paths were never tested, right?)
One of the biggest TTM reworks was to introduce the atomicity assumption 
and remove a lot of code that was
prone to deadlocks, races and buffer leaks. I'm not prepared to revert 
that work without an extremely
good reason, and "It can be done" is not such a reason.

We *need* to carefully weigh it against any benefits you have in your 
work, and you need to test these codepaths
in parallell cases subject to heavy aperture / vram thrashing and 
frequent signals causing interrupted waits.

And I think you need to present the gains in your work that can motivate 
the testing-and review time for this.

Thanks,
Thomas





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