Message-ID: <50696699.7020009@canonical.com>
Date:	Mon, 01 Oct 2012 11:47:05 +0200
From:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
To:	Thomas Hellstrom <thellstrom@...are.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

Op 28-09-12 21:42, Thomas Hellstrom schreef:
> 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.
I was doing a evil hack where I 'released' lru_lock to lockdep before doing the annotation
for a blocking acquire, and left trylock annotations as they were. This made lockdep do the
right thing.
> 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?
l thought list_kref made sure of that? Even if not the only one with a reference, the list_empty check would
make sure it would only run once. I'l fix it up again so it doesn't become a WARN_ON_ONCE, I didn't know
it could run multiple times at the time, so I'll change it back to unlikely.

> 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.
Deepest apologies, I only did a quick glance at the code part of this email, overlooked this part since
I was swamped with other things and meant to do a full reply on monday. I didn't mean to make it sound
like I only cared blindly about merging my code, just wanted to find a good solution.
> 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.
Agreed, is there already a tester for this or should I write my own?
> And I think you need to present the gains in your work that can motivate the testing-and review time for this.
Agreed.

~Maarten
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