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@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 > 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 _______________________________________________ amd-gfx mailing list amd-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/amd-gfx
