syzbot

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.0.184/6534 is trying to acquire lock:
ffff8880b8923d90 ((softirq_ctrl.lock)){+.+.}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:44 [inline]
ffff8880b8923d90 ((softirq_ctrl.lock)){+.+.}-{3:3}, at: __local_bh_disable_ip+0x264/0x400 kernel/softirq.c:168

but task is already holding lock:
ffff8880242183a0 (&dev->vblank_time_lock){+.+.}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:44 [inline]
ffff8880242183a0 (&dev->vblank_time_lock){+.+.}-{3:3}, at: drm_vblank_disable_and_save+0x7f/0x380 drivers/gpu/drm/drm_vblank.c:463

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #4 (&dev->vblank_time_lock){+.+.}-{3:3}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       rt_spin_lock+0x88/0x2c0 kernel/locking/spinlock_rt.c:56
       spin_lock include/linux/spinlock_rt.h:44 [inline]
       drm_reset_vblank_timestamp drivers/gpu/drm/drm_vblank.c:246 [inline]
       drm_crtc_vblank_on_config+0x2cd/0x860 drivers/gpu/drm/drm_vblank.c:1504
       drm_crtc_vblank_on+0x88/0xc0 drivers/gpu/drm/drm_vblank.c:1535
       crtc_enable drivers/gpu/drm/drm_atomic_helper.c:1610 [inline]
       drm_atomic_helper_commit_modeset_enables+0x602/0xe10 drivers/gpu/drm/drm_atomic_helper.c:1680
       vkms_atomic_commit_tail+0x69/0x210 drivers/gpu/drm/vkms/vkms_drv.c:65
       commit_tail+0x281/0x3a0 drivers/gpu/drm/drm_atomic_helper.c:1986
       drm_atomic_helper_commit+0xa6b/0xb10 drivers/gpu/drm/drm_atomic_helper.c:2226
       drm_atomic_commit+0x262/0x2c0 drivers/gpu/drm/drm_atomic.c:1577
       drm_client_modeset_commit_atomic+0x620/0x760 drivers/gpu/drm/drm_client_modeset.c:1103
       drm_client_modeset_commit_locked+0xce/0x4d0 drivers/gpu/drm/drm_client_modeset.c:1206
       drm_client_modeset_commit+0x4a/0x70 drivers/gpu/drm/drm_client_modeset.c:1232
       __drm_fb_helper_restore_fbdev_mode_unlocked+0x9d/0x1b0 drivers/gpu/drm/drm_fb_helper.c:238
       drm_fb_helper_set_par+0xaf/0x100 drivers/gpu/drm/drm_fb_helper.c:1360
       fbcon_init+0x1255/0x2370 drivers/video/fbdev/core/fbcon.c:1130
       visual_init+0x2ef/0x650 drivers/tty/vt/vt.c:1019
       do_bind_con_driver+0x890/0xf70 drivers/tty/vt/vt.c:3915
       do_take_over_console+0x899/0xa10 drivers/tty/vt/vt.c:4481
       do_fbcon_takeover+0x118/0x200 drivers/video/fbdev/core/fbcon.c:565
       do_fb_registered drivers/video/fbdev/core/fbcon.c:3008 [inline]
       fbcon_fb_registered+0x35e/0x610 drivers/video/fbdev/core/fbcon.c:3028
       do_register_framebuffer drivers/video/fbdev/core/fbmem.c:512 [inline]
       register_framebuffer+0x70f/0x890 drivers/video/fbdev/core/fbmem.c:578
       __drm_fb_helper_initial_config_and_unlock+0x130a/0x18a0 drivers/gpu/drm/drm_fb_helper.c:1852
       drm_fbdev_client_hotplug+0x16f/0x230 drivers/gpu/drm/clients/drm_fbdev_client.c:52
       drm_client_register+0x16f/0x210 drivers/gpu/drm/drm_client.c:141
       drm_fbdev_client_setup+0x19f/0x3f0 drivers/gpu/drm/clients/drm_fbdev_client.c:159
       drm_client_setup+0x10a/0x230 drivers/gpu/drm/clients/drm_client_setup.c:46
       vkms_create drivers/gpu/drm/vkms/vkms_drv.c:201 [inline]
       vkms_init+0x3e0/0x4b0 drivers/gpu/drm/vkms/vkms_drv.c:221
       do_one_initcall+0x233/0x820 init/main.c:1269
       do_initcall_level+0x104/0x190 init/main.c:1331
       do_initcalls+0x59/0xa0 init/main.c:1347
       kernel_init_freeable+0x334/0x4b0 init/main.c:1579
       kernel_init+0x1d/0x1d0 init/main.c:1469
       ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #3 (&dev->vbl_lock){+.+.}-{3:3}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       rt_spin_lock+0x88/0x2c0 kernel/locking/spinlock_rt.c:56
       spin_lock include/linux/spinlock_rt.h:44 [inline]
       vblank_disable_fn+0x72/0x190 drivers/gpu/drm/drm_vblank.c:496
       call_timer_fn+0x17b/0x5f0 kernel/time/timer.c:1747
       expire_timers kernel/time/timer.c:1798 [inline]
       __run_timers kernel/time/timer.c:2372 [inline]
       __run_timer_base+0x648/0x970 kernel/time/timer.c:2384
       run_timer_base kernel/time/timer.c:2393 [inline]
       run_timer_softirq+0xb7/0x180 kernel/time/timer.c:2403
       handle_softirqs+0x22c/0x710 kernel/softirq.c:579
       __do_softirq kernel/softirq.c:613 [inline]
       run_ktimerd+0xcf/0x190 kernel/softirq.c:1043
       smpboot_thread_fn+0x542/0xa60 kernel/smpboot.c:160
       kthread+0x711/0x8a0 kernel/kthread.c:463
       ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #2 ((&vblank->disable_timer)){+...}-{0:0}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       call_timer_fn+0xdb/0x5f0 kernel/time/timer.c:1744
       expire_timers kernel/time/timer.c:1798 [inline]
       __run_timers kernel/time/timer.c:2372 [inline]
       __run_timer_base+0x648/0x970 kernel/time/timer.c:2384
       run_timer_base kernel/time/timer.c:2393 [inline]
       run_timer_softirq+0xb7/0x180 kernel/time/timer.c:2403
       handle_softirqs+0x22c/0x710 kernel/softirq.c:579
       __do_softirq kernel/softirq.c:613 [inline]
       run_ktimerd+0xcf/0x190 kernel/softirq.c:1043
       smpboot_thread_fn+0x542/0xa60 kernel/smpboot.c:160
       kthread+0x711/0x8a0 kernel/kthread.c:463
       ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #1 (&base->expiry_lock){+...}-{3:3}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       rt_spin_lock+0x88/0x2c0 kernel/locking/spinlock_rt.c:56
       spin_lock include/linux/spinlock_rt.h:44 [inline]
       timer_base_lock_expiry kernel/time/timer.c:1501 [inline]
       __run_timer_base+0x114/0x970 kernel/time/timer.c:2382
       run_timer_base kernel/time/timer.c:2393 [inline]
       run_timer_softirq+0x67/0x180 kernel/time/timer.c:2401
       handle_softirqs+0x22c/0x710 kernel/softirq.c:579
       __do_softirq kernel/softirq.c:613 [inline]
       run_ktimerd+0xcf/0x190 kernel/softirq.c:1043
       smpboot_thread_fn+0x542/0xa60 kernel/smpboot.c:160
       kthread+0x711/0x8a0 kernel/kthread.c:463
       ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #0 ((softirq_ctrl.lock)){+.+.}-{3:3}:
       check_prev_add kernel/locking/lockdep.c:3165 [inline]
       check_prevs_add kernel/locking/lockdep.c:3284 [inline]
       validate_chain+0xb9b/0x2140 kernel/locking/lockdep.c:3908
       __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5237
       reacquire_held_locks+0x127/0x1d0 kernel/locking/lockdep.c:5385
       __lock_release kernel/locking/lockdep.c:5574 [inline]
       lock_release+0x1b4/0x3e0 kernel/locking/lockdep.c:5889
       __local_bh_enable_ip+0x10c/0x270 kernel/softirq.c:228
       hrtimer_cancel+0x39/0x60 kernel/time/hrtimer.c:1491
       __disable_vblank drivers/gpu/drm/drm_vblank.c:442 [inline]
       drm_vblank_disable_and_save+0x1bc/0x380 drivers/gpu/drm/drm_vblank.c:481
       drm_crtc_vblank_off+0x22e/0x820 drivers/gpu/drm/drm_vblank.c:1364
       crtc_disable drivers/gpu/drm/drm_atomic_helper.c:1258 [inline]
       disable_outputs drivers/gpu/drm/drm_atomic_helper.c:1340 [inline]
       drm_atomic_helper_commit_modeset_disables+0xc89/0x2010 drivers/gpu/drm/drm_atomic_helper.c:1524
       vkms_atomic_commit_tail+0x51/0x210 drivers/gpu/drm/vkms/vkms_drv.c:61
       commit_tail+0x281/0x3a0 drivers/gpu/drm/drm_atomic_helper.c:1986
       drm_atomic_helper_commit+0xa6b/0xb10 drivers/gpu/drm/drm_atomic_helper.c:2226
       drm_atomic_commit+0x262/0x2c0 drivers/gpu/drm/drm_atomic.c:1577
       drm_atomic_helper_set_config+0xe2/0x160 drivers/gpu/drm/drm_atomic_helper.c:3426
       drm_mode_setcrtc+0x9a4/0x1c50 drivers/gpu/drm/drm_crtc.c:869
       drm_ioctl_kernel+0x2d2/0x3a0 drivers/gpu/drm/drm_ioctl.c:796
       drm_ioctl+0x685/0xb20 drivers/gpu/drm/drm_ioctl.c:893
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:598 [inline]
       __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:584
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
  (softirq_ctrl.lock) --> &dev->vbl_lock --> &dev->vblank_time_lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&dev->vblank_time_lock);
                               lock(&dev->vbl_lock);
                               lock(&dev->vblank_time_lock);
  lock((softirq_ctrl.lock));

 *** DEADLOCK ***

8 locks held by syz.0.184/6534:
 #0: ffffc900052d7b20 (crtc_ww_class_acquire){+.+.}-{0:0}, at: drm_mode_setcrtc+0x555/0x1c50 drivers/gpu/drm/drm_crtc.c:730
 #1: ffffc900052d7b48 (crtc_ww_class_mutex){+.+.}-{4:4}, at: drm_mode_setcrtc+0x555/0x1c50 drivers/gpu/drm/drm_crtc.c:730
 #2: ffff8880242184b8 (&dev->event_lock){+.+.}-{3:3}, at: spin_lock_irq include/linux/spinlock_rt.h:93 [inline]
 #2: ffff8880242184b8 (&dev->event_lock){+.+.}-{3:3}, at: drm_crtc_vblank_off+0xe4/0x820 drivers/gpu/drm/drm_vblank.c:1355
 #3: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:331 [inline]
 #3: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:841 [inline]
 #3: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: __rt_spin_lock kernel/locking/spinlock_rt.c:50 [inline]
 #3: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: rt_spin_lock+0x1bb/0x2c0 kernel/locking/spinlock_rt.c:57
 #4: ffff888024218420 (&dev->vbl_lock){+.+.}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:44 [inline]
 #4: ffff888024218420 (&dev->vbl_lock){+.+.}-{3:3}, at: drm_crtc_vblank_off+0xf5/0x820 drivers/gpu/drm/drm_vblank.c:1357
 #5: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:331 [inline]
 #5: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:841 [inline]
 #5: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: __rt_spin_lock kernel/locking/spinlock_rt.c:50 [inline]
 #5: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: rt_spin_lock+0x1bb/0x2c0 kernel/locking/spinlock_rt.c:57
 #6: ffff8880242183a0 (&dev->vblank_time_lock){+.+.}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:44 [inline]
 #6: ffff8880242183a0 (&dev->vblank_time_lock){+.+.}-{3:3}, at: drm_vblank_disable_and_save+0x7f/0x380 drivers/gpu/drm/drm_vblank.c:463
 #7: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:331 [inline]
 #7: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:841 [inline]
 #7: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: __rt_spin_lock kernel/locking/spinlock_rt.c:50 [inline]
 #7: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: rt_spin_lock+0x1bb/0x2c0 kernel/locking/spinlock_rt.c:57

stack backtrace:
CPU: 1 UID: 0 PID: 6534 Comm: syz.0.184 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
 print_circular_bug+0x2ee/0x310 kernel/locking/lockdep.c:2043
 check_noncircular+0x134/0x160 kernel/locking/lockdep.c:2175
 check_prev_add kernel/locking/lockdep.c:3165 [inline]
 check_prevs_add kernel/locking/lockdep.c:3284 [inline]
 validate_chain+0xb9b/0x2140 kernel/locking/lockdep.c:3908
 __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5237
 reacquire_held_locks+0x127/0x1d0 kernel/locking/lockdep.c:5385
 __lock_release kernel/locking/lockdep.c:5574 [inline]
 lock_release+0x1b4/0x3e0 kernel/locking/lockdep.c:5889
 __local_bh_enable_ip+0x10c/0x270 kernel/softirq.c:228
 hrtimer_cancel+0x39/0x60 kernel/time/hrtimer.c:1491
 __disable_vblank drivers/gpu/drm/drm_vblank.c:442 [inline]
 drm_vblank_disable_and_save+0x1bc/0x380 drivers/gpu/drm/drm_vblank.c:481
 drm_crtc_vblank_off+0x22e/0x820 drivers/gpu/drm/drm_vblank.c:1364
 crtc_disable drivers/gpu/drm/drm_atomic_helper.c:1258 [inline]
 disable_outputs drivers/gpu/drm/drm_atomic_helper.c:1340 [inline]
 drm_atomic_helper_commit_modeset_disables+0xc89/0x2010 drivers/gpu/drm/drm_atomic_helper.c:1524
 vkms_atomic_commit_tail+0x51/0x210 drivers/gpu/drm/vkms/vkms_drv.c:61
 commit_tail+0x281/0x3a0 drivers/gpu/drm/drm_atomic_helper.c:1986
 drm_atomic_helper_commit+0xa6b/0xb10 drivers/gpu/drm/drm_atomic_helper.c:2226
 drm_atomic_commit+0x262/0x2c0 drivers/gpu/drm/drm_atomic.c:1577
 drm_atomic_helper_set_config+0xe2/0x160 drivers/gpu/drm/drm_atomic_helper.c:3426
 drm_mode_setcrtc+0x9a4/0x1c50 drivers/gpu/drm/drm_crtc.c:869
 drm_ioctl_kernel+0x2d2/0x3a0 drivers/gpu/drm/drm_ioctl.c:796
 drm_ioctl+0x685/0xb20 drivers/gpu/drm/drm_ioctl.c:893
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:598 [inline]
 __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:584
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fcd2143ebe9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fcd20a85038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fcd21666090 RCX: 00007fcd2143ebe9
RDX: 0000200000000740 RSI: 00000000c06864a2 RDI: 0000000000000003
RBP: 00007fcd214c1e19 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fcd21666128 R14: 00007fcd21666090 R15: 00007ffd85f10678
 </TASK>