Hi all, for all = rcu, cpuhotplug and perf maintainers
We've hit an interesting new lockdep splat in our drm/i915 CI:
https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_17096/shard-tglb7/igt@kms...
Summarizing away the driver parts we have
< gpu locks which are held within mm->mmap_sem in various gpu fault handlers >
-> #4 (&mm->mmap_sem#2){++++}: <4> [604.892615] __might_fault+0x63/0x90 <4> [604.892617] _copy_to_user+0x1e/0x80 <4> [604.892619] perf_read+0x200/0x2b0 <4> [604.892621] vfs_read+0x96/0x160 <4> [604.892622] ksys_read+0x9f/0xe0 <4> [604.892623] do_syscall_64+0x4f/0x220 <4> [604.892624] entry_SYSCALL_64_after_hwframe+0x49/0xbe <4> [604.892625] -> #3 (&cpuctx_mutex){+.+.}: <4> [604.892626] __mutex_lock+0x9a/0x9c0 <4> [604.892627] perf_event_init_cpu+0xa4/0x140 <4> [604.892629] perf_event_init+0x19d/0x1cd <4> [604.892630] start_kernel+0x362/0x4e4 <4> [604.892631] secondary_startup_64+0xa4/0xb0 <4> [604.892631] -> #2 (pmus_lock){+.+.}: <4> [604.892633] __mutex_lock+0x9a/0x9c0 <4> [604.892633] perf_event_init_cpu+0x6b/0x140 <4> [604.892635] cpuhp_invoke_callback+0x9b/0x9d0 <4> [604.892636] _cpu_up+0xa2/0x140 <4> [604.892637] do_cpu_up+0x61/0xa0 <4> [604.892639] smp_init+0x57/0x96 <4> [604.892639] kernel_init_freeable+0x87/0x1dc <4> [604.892640] kernel_init+0x5/0x100 <4> [604.892642] ret_from_fork+0x24/0x50 <4> [604.892642] -> #1 (cpu_hotplug_lock.rw_sem){++++}: <4> [604.892643] cpus_read_lock+0x34/0xd0 <4> [604.892644] rcu_barrier+0xaa/0x190 <4> [604.892645] kernel_init+0x21/0x100 <4> [604.892647] ret_from_fork+0x24/0x50 <4> [604.892647] -> #0 (rcu_state.barrier_mutex){+.+.}: <4> [604.892649] __lock_acquire+0x1328/0x15d0 <4> [604.892650] lock_acquire+0xa7/0x1c0 <4> [604.892651] __mutex_lock+0x9a/0x9c0 <4> [604.892652] rcu_barrier+0x23/0x190 <4> [604.892680] i915_gem_object_unbind+0x29d/0x3f0 [i915] <4> [604.892707] i915_gem_object_pin_to_display_plane+0x141/0x270 [i915] <4> [604.892737] intel_pin_and_fence_fb_obj+0xec/0x1f0 [i915] <4> [604.892767] intel_plane_pin_fb+0x3f/0xd0 [i915] <4> [604.892797] intel_prepare_plane_fb+0x13b/0x5c0 [i915] <4> [604.892798] drm_atomic_helper_prepare_planes+0x85/0x110 <4> [604.892827] intel_atomic_commit+0xda/0x390 [i915] <4> [604.892828] drm_atomic_helper_set_config+0x57/0xa0 <4> [604.892830] drm_mode_setcrtc+0x1c4/0x720 <4> [604.892830] drm_ioctl_kernel+0xb0/0xf0 <4> [604.892831] drm_ioctl+0x2e1/0x390 <4> [604.892833] ksys_ioctl+0x7b/0x90 <4> [604.892835] __x64_sys_ioctl+0x11/0x20 <4> [604.892835] do_syscall_64+0x4f/0x220 <4> [604.892836] entry_SYSCALL_64_after_hwframe+0x49/0xbe
The last backtrace boils down to i915 driver code which holds the same locks we are holding within mm->mmap_sem, and then ends up calling rcu_barrier(). From what I can see i915 is just the messenger here, any driver with this pattern of a lock held within mmap_sem which also has a path of calling rcu_barrier while holding that lock should be hitting this splat.
Two questions: - This suggests that calling rcu_barrier() isn't ok anymore while holding mmap_sem, or anything that has a dependency upon mmap_sem. I guess that's not the idea, please confirm. - Assuming this depedency is indeed not intended, where should the loop be broken? It goes through perf, cpuhotplug and rcu subsystems, and I don't have a clue about any of those.
Thanks a lot.
Cheers, Daniel