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@xxxxxxx> >>> Cc: Harry Wentland <harry.wentland@xxxxxxx> >>> Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@xxxxxxx> >> >> 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@xxxxxxxxxxxxxxxxxxxxx > 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@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/amd-gfx
