[PATCH] drm/amd/display: Add fast path for cursor plane updates

[Kazlauskas, Nicholas]

On 12/13/18 2:21 PM, Kazlauskas, Nicholas wrote:
> On 12/13/18 11:01 AM, Kazlauskas, Nicholas wrote:
>> On 12/13/18 10:48 AM, Michel Dänzer wrote:
>>> On 2018-12-05 8:59 p.m., Nicholas Kazlauskas wrote:
>>>> [Why]
>>>> Legacy cursor plane updates from drm helpers go through the full
>>>> atomic codepath. A high volume of cursor updates through this slow
>>>> code path can cause subsequent page-flips to skip vblank intervals
>>>> since each individual update is slow.
>>>>
>>>> This problem is particularly noticeable for the compton compositor.
>>>>
>>>> [How]
>>>> A fast path for cursor plane updates is added by using DRM asynchronous
>>>> commit support provided by async_check and async_update. These don't do
>>>> a full state/flip_done dependency stall and they don't block other
>>>> commit work.
>>>>
>>>> However, DC still expects itself to be single-threaded for anything
>>>> that can issue register writes. Screen corruption or hangs can occur
>>>> if write sequences overlap. Every call that potentially perform
>>>> register writes needs to be guarded for asynchronous updates to work.
>>>> The dc_lock mutex was added for this.
>>>>
>>>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106175
>>>>
>>>> Cc: Leo Li <sunpeng.li at amd.com>
>>>> Cc: Harry Wentland <harry.wentland at amd.com>
>>>> Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas at amd.com>
>>>
>>> Looks like this change introduced (or at least exposed) a reference
>>> counting bug resulting in use-after-free when Xorg shuts down[0]. See
>>> the attached dmesg excerpt (note that I wrapped the !bo->pin_count check
>>> in amdgpu_bo_unpin in WARN_ON_ONCE).
>>>
>>>
>>> [0] Only with
>>> https://gitlab.freedesktop.org/xorg/driver/xf86-video-amdgpu/commit/0d60233d26ec70d4e1faa343b438e33829c6d5e4
>>> , i.e. alternating between two BOs for the HW cursor, instead of always
>>> using the same one.
>>>
>>>
>>
>> Thanks for the heads up, I don't think I had that patch in my
>> xf86-video-amdgpu when testing the desktop stack.
>>
>> The async atomic helper does the:
>>
>> drm_atomic_helper_prepare_planes
>> drm_atomic_helper_async_commit
>> drm_atomic_helper_cleanup_planes
>>
>> ...sequence correctly from what I can tell, so maybe it's something with
>> dm_plane_helper_prepare_fb or dm_plane_helper_cleanup_fb itself.
>>
>> One case where unref could be called (not following a ref) is during
>> drm_atomic_helper_prepare_planes - if prepare_fb fails then cleanup_fb
>> gets called regardless, and we only ref the fb if prepare_fb is in the
>> success path.
>>
>> Nicholas Kazlauskas
>> _______________________________________________
>> amd-gfx mailing list
>> amd-gfx at lists.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/amd-gfx
>>
> 
> The prepare_fb/cleanup_fb calls are actually fine since cleanup_fb only
> gets called on planes that had prepare_fb succeed in all cases as far as
> I can tell.
> 
> I think the bug here might be forgetting to set the plane->state to the
> new_state. The cleanup fb callback decides whether to call it on the old
> plane state or new plane state depending on if the commit was aborted or
> not. I think every fast plane update might be treated as aborted in this
> case.
> 
> Nicholas Kazlauskas
> _______________________________________________
> amd-gfx mailing list
> amd-gfx at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/amd-gfx
> 

This is a bug with DRM, actually.

Typically for a regular atomic commit the prepare_fb callback is called 
for the new_plane_state and cleanup_fb is called for the old_plane_state 
at the end of commit tail.

However, for asynchronous commits this isn't the same - prepare_fb is 
called for new_plane_state and cleanup_fb is then immediately called 
after also for the new_plane_state.

Looking at your stack trace I can see that this is exactly what causes 
the use after free,

The CRTC has changed so it's stuck in the slow path (commit_tail is in 
the trace). However, the plane->state->fb has already been unpinned and 
unref. But the plane->state->fb is *not* NULL from the previous fast 
update, so when it gets to cleanup planes it tries to free the 
old_plane_state it unpins and unrefs the bo a second time.

Then a new fast cursor update comes along (and the fb hasn't changed) so 
it tries to prepare_fb on the same freed bo.

Nicholas Kazlauskas