On Tue, Dec 5, 2023 at 8:56 AM Rob Clark robdclark@gmail.com wrote:
On Tue, Dec 5, 2023 at 7:58 AM Christian König christian.koenig@amd.com wrote:
Am 05.12.23 um 16:41 schrieb Rob Clark:
On Mon, Dec 4, 2023 at 10:46 PM Christian König christian.koenig@amd.com wrote:
Am 04.12.23 um 22:54 schrieb Rob Clark:
On Thu, Mar 23, 2023 at 2:30 PM Rob Clark robdclark@gmail.com 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..
Oh, hmm, _add_dependency() is consuming the fence reference
BR, -R
BR, -R
Thanks, Christian.
BR, -R
Regards, Christian.
BR, -R