Re: [PATCH 04/10] drm/ttm: change fence_lock to inner lock, v3

[Maarten Lankhorst <m.b.lankhorst@gmail.com>]

Op 19-11-12 16:04, Thomas Hellstrom schreef:
> On 11/19/2012 03:17 PM, Thomas Hellstrom wrote:
>> Hi,
>>
>> This patch looks mostly good, although I think ttm_bo_cleanup_refs becomes overly complicated:
>> Could this do, or am I missing something?
>>
>
> Actually, my version is bad, because ttm_bo_wait() is called with the lru lock held.
>
> /Thomas
Oh digging through it made me remember why I had to release the reservation early and
had to allow move_notify to be called without reservation.

Fortunately move_notify has a NULL parameter, which is the only time that happens,
so you can still check do BUG_ON(mem != NULL && !ttm_bo_reserved(bo)); in your
move_notify handler.

05/10 removed the loop and assumed no new fence could be attached after the driver has
declared the bo dead.

However, at that point it may no longer hold a reservation to confirm this, that's why
I moved the cleanup to be done in the release_list handler. It could still be done in
ttm_bo_release, but we no longer have a reservation after we waited. Getting
a reservation can fail if the bo is imported for example.

While it would be true that in that case a new fence may be attached as well, that
would be less harmful since that operation wouldn't involve this device, so the
ttm bo can still be removed in that case. When that time comes I should probably
fix up that WARN_ON(ret) in ttm_bo_cleanup_refs. :-)

I did add a WARN_ON(!atomic_read(&bo->kref.refcount)); to
ttm_bo_reserve and ttm_eu_reserve_buffers to be sure nothing is done on the device
itself. If that is too paranoid, those WARN_ON's could be dropped. I prefer to leave them
in for a kernel release or 2. But according to the rules that would be the only time you
could attach a new fence and trigger the WARN_ON for now..

~Maarten