On 5/20/26 10:12, Dmitry Osipenko wrote: > On 5/20/26 10:05, Christian König wrote: >> On 5/20/26 08:50, Dmitry Osipenko wrote: >>> On 5/19/26 11:27, Christian König wrote: >>>> On 5/19/26 10:22, Deepanshu Kartikey wrote: >>>>> virtio_gpu_cursor_plane_update() and virtio_gpu_resource_flush() lock >>>>> the framebuffer BO's dma_resv via virtio_gpu_array_lock_resv() and >>>>> ignore its return value. The function can fail with -EINTR from >>>>> dma_resv_lock_interruptible() (signal during lock wait) or with >>>>> -ENOMEM from dma_resv_reserve_fences() (fence slot allocation), >>>>> leaving the resv lock not held. The queue path then walks the object >>>>> array and calls dma_resv_add_fence(), which requires the lock held; >>>>> with lockdep enabled this trips dma_resv_assert_held(): >>>>> >>>>> WARNING: drivers/dma-buf/dma-resv.c:296 at >>>>> dma_resv_add_fence+0x71e/0x840 >>>>> Call Trace: >>>>> virtio_gpu_array_add_fence >>>>> virtio_gpu_queue_ctrl_sgs >>>>> virtio_gpu_queue_fenced_ctrl_buffer >>>>> virtio_gpu_cursor_plane_update >>>>> drm_atomic_helper_commit_planes >>>>> drm_atomic_helper_commit_tail >>>>> commit_tail >>>>> drm_atomic_helper_commit >>>>> drm_atomic_commit >>>>> drm_atomic_helper_update_plane >>>>> __setplane_atomic >>>>> drm_mode_cursor_universal >>>>> drm_mode_cursor_common >>>>> drm_mode_cursor_ioctl >>>>> drm_ioctl >>>>> __x64_sys_ioctl >>>>> >>>>> Beyond the WARN, mutating the dma_resv fence list without the lock >>>>> races with concurrent readers/writers and can corrupt the list. >>>> >>>> Well why are you trying to add a fence on an atomic mode set in the first >>>> place? >>>> >>>> That is usually an illegal operation here. >>> That is pre-existing in the driver. It performs draw operation and in >>> some cases waits for the completion during atomic. Whether all that >>> syncing is correct is hard to say immediately as some of it may be >>> historical edge cases. >> >> I'm not not so deeply in the atomic mode setting stuff but it strongly >> sounds like that this is seriously broken. >> >> The background is that the atomic mode set framework allows an output >> dma_fence which is signaled when the commit is finished. >> >> So when you allocate a fence slot and add a new fence to finish the atomic >> commit it is trivially possible that this cycles back and waits for the >> atomic commit to finish. In other words you have a deadlock. >> >> You probably need specially crafted userspace with the right timing to >> trigger that, but such issues are usually a rather big no-no and need to be >> fixed in the long term. >> >> Try to add dma_fence_begin_signaling() and dma_fence_end_signaling() >> annotation and enable lockdep, the tool should be able to point out if and >> what exactly goes wrong. >> >> The usual fix is to prepare everything before commit_tail is called (alloc >> memory, create, reserve slot, add dma_fence etc....) and then just send out >> the prepared commands later on. > > We tried with moving resv alloc to prepare_fb() in a previous patch > version, it resulted in a non-trivial deadlocks. The goal of this patch > is to fix immediate problem with a minimal code change.
Yeah, totally fine with me to get that fixed first. > What you're saying is correct, but it may require a rather big > refactoring of the code. In general, everything works okay today, so not > really an urgent problem. It's just a potential issue and when the AI bots keep evolving like they already do they will sooner or later start to point that out as well. Regards, Christian.
