On Tue, Dec 5, 2023 at 7:58 AM Christian König <christian.koenig@xxxxxxx> wrote: > > Am 05.12.23 um 16:41 schrieb Rob Clark: > > On Mon, Dec 4, 2023 at 10:46 PM Christian König > > <christian.koenig@xxxxxxx> wrote: > >> Am 04.12.23 um 22:54 schrieb Rob Clark: > >>> On Thu, Mar 23, 2023 at 2:30 PM Rob Clark <robdclark@xxxxxxxxx> wrote: > >>>> [SNIP] > >>> So, this patch turns out to blow up spectacularly with dma_fence > >>> refcnt underflows when I enable DRIVER_SYNCOBJ_TIMELINE .. I think, > >>> because it starts unwrapping fence chains, possibly in parallel with > >>> fence signaling on the retire path. Is it supposed to be permissible > >>> to unwrap a fence chain concurrently? > >> The DMA-fence chain object and helper functions were designed so that > >> concurrent accesses to all elements are always possible. > >> > >> See dma_fence_chain_walk() and dma_fence_chain_get_prev() for example. > >> dma_fence_chain_walk() starts with a reference to the current fence (the > >> anchor of the walk) and tries to grab an up to date reference on the > >> previous fence in the chain. Only after that reference is successfully > >> acquired we drop the reference to the anchor where we started. > >> > >> Same for dma_fence_array_first(), dma_fence_array_next(). Here we hold a > >> reference to the array which in turn holds references to each fence > >> inside the array until it is destroyed itself. > >> > >> When this blows up we have somehow mixed up the references somewhere. > > That's what it looked like to me, but wanted to make sure I wasn't > > overlooking something subtle. And in this case, the fence actually > > should be the syncobj timeline point fence, not the fence chain. > > Virtgpu has essentially the same logic (there we really do want to > > unwrap fences so we can pass host fences back to host rather than > > waiting in guest), I'm not sure if it would blow up in the same way. > > Well do you have a backtrace of what exactly happens? > > Maybe we have some _put() before _get() or something like this. I hacked up something to store the backtrace in dma_fence_release() (and leak the block so the backtrace would still be around later when dma_fence_get/put was later called) and ended up with: [ 152.811360] freed at: [ 152.813718] dma_fence_release+0x30/0x134 [ 152.817865] dma_fence_put+0x38/0x98 [gpu_sched] [ 152.822657] drm_sched_job_add_dependency+0x160/0x18c [gpu_sched] [ 152.828948] drm_sched_job_add_syncobj_dependency+0x58/0x88 [gpu_sched] [ 152.835770] msm_ioctl_gem_submit+0x580/0x1160 [msm] [ 152.841070] drm_ioctl_kernel+0xec/0x16c [ 152.845132] drm_ioctl+0x2e8/0x3f4 [ 152.848646] vfs_ioctl+0x30/0x50 [ 152.851982] __arm64_sys_ioctl+0x80/0xb4 [ 152.856039] invoke_syscall+0x8c/0x120 [ 152.859919] el0_svc_common.constprop.0+0xc0/0xdc [ 152.864777] do_el0_svc+0x24/0x30 [ 152.868207] el0_svc+0x8c/0xd8 [ 152.871365] el0t_64_sync_handler+0x84/0x12c [ 152.875771] el0t_64_sync+0x190/0x194 I suppose that doesn't guarantee that this was the problematic put. But dropping this patch to unwrap the fence makes the problem go away.. BR, -R > Thanks, > Christian. > > > > > BR, > > -R > > > >> Regards, > >> Christian. > >> > >>> BR, > >>> -R >
