[BUG] RCU stall in vkms_vblank_simulate/drm_handle_vblank during kcov_ioctl (6.18.0)

[王志]

Dear Developers,
I am reporting an RCU CPU stall on Linux 6.18.0 detected by Syzkaller. The stall occurs during the interaction between kcov memory allocation and vkms vblank simulation. 

Issue Summary: CPU 0 is interrupted by a timer while performing vmalloc_user for kcov. The hrtimer interrupt executes vkms_vblank_simulate, which then stalls in drm_handle_vblank waiting for a spinlock. Meanwhile, CPU 1 is in drm_file_free, likely holding the lock. 

Stack Trace Snippet:
RIP: native_queued_spin_lock_slowpath
Call Trace:
 <IRQ>
 drm_handle_vblank+0x132/0xc70
 vkms_vblank_simulate+0xa8/0x390
 hrtimer_interrupt
 <TASK>
 kcov_ioctl+0x4c/0x6f0
Environment:
Kernel: 6.18.0 PREEMPT(full)
Arch: x86_64 (QEMU)
Full log is attached. Please let me know if a reproducer is needed.
rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu: 	1-...!: (3 ticks this GP) idle=8004/1/0x4000000000000000 softirq=244774/244774 fqs=4
rcu: 	(detected by 2, t=10502 jiffies, g=357853, q=787 ncpus=4)
Sending NMI from CPU 2 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 UID: 0 PID: 608 Comm: kworker/u17:5 Not tainted 6.18.0 #1 PREEMPT(full) 
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
Workqueue: events_unbound toggle_allocation_gate
RIP: 0010:native_queued_spin_lock_slowpath+0x23e/0x9c0
Code: 02 48 89 e8 83 e0 07 83 c0 01 38 d0 7c 08 84 d2 0f 85 1c 07 00 00 b8 01 00 00 00 66 89 45 00 e9 c2 fe ff ff 89 44 24 40 f3 90 <e9> 5e fe ff ff 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03
RSP: 0018:ffffc900001f8b78 EFLAGS: 00000002
RAX: 0000000000000001 RBX: 0000000000000001 RCX: ffffffff8b43f32e
RDX: ffffed102099446b RSI: 0000000000000004 RDI: ffff888104ca2350
RBP: ffff888104ca2350 R08: 0000000000000000 R09: ffffed102099446a
R10: ffff888104ca2353 R11: 0000000000000000 R12: 1ffff9200003f171
R13: 0000000000000003 R14: ffffed102099446a R15: ffffc900001f8bb8
FS:  0000000000000000(0000) GS:ffff8881a2601000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffdbbfd3dd8 CR3: 000000000df84000 CR4: 00000000000006f0
Call Trace:
 <IRQ>
 debug_spin_lock_before home/wmy/Fuzzer/third_tool/linux-6.18/kernel/locking/spinlock_debug.c:87 [inline]
 do_raw_spin_lock+0x20d/0x2b0 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/locking/spinlock_debug.c:115
 __raw_spin_lock_irqsave home/wmy/Fuzzer/third_tool/linux-6.18/include/linux/spinlock_api_smp.h:110 [inline]
 _raw_spin_lock_irqsave+0x45/0x60 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/locking/spinlock.c:162
 drm_handle_vblank+0x125/0xc70
 vkms_vblank_simulate+0xa8/0x390
 __run_hrtimer home/wmy/Fuzzer/third_tool/linux-6.18/kernel/time/hrtimer.c:1779 [inline]
 __hrtimer_run_queues+0x1f5/0xb30 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/time/hrtimer.c:1841
 hrtimer_interrupt+0x39a/0x880 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/time/hrtimer.c:1912
 instrument_atomic_read home/wmy/Fuzzer/third_tool/linux-6.18/include/linux/instrumented.h:68 [inline]
 _test_bit home/wmy/Fuzzer/third_tool/linux-6.18/include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]
 cpumask_test_cpu home/wmy/Fuzzer/third_tool/linux-6.18/include/linux/cpumask.h:646 [inline]
 cpu_online home/wmy/Fuzzer/third_tool/linux-6.18/include/linux/cpumask.h:1205 [inline]
 __do_trace_local_timer_exit home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/include/asm/trace/irq_vectors.h:40 [inline]
 trace_local_timer_exit home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/include/asm/trace/irq_vectors.h:40 [inline]
 __sysvec_apic_timer_interrupt+0x10d/0x400 home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/kernel/apic/apic.c:1059
 sysvec_apic_timer_interrupt+0xa3/0xc0 home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/kernel/apic/apic.c:2145
 </IRQ>
 <TASK>
 asm_sysvec_apic_timer_interrupt+0x1a/0x20 home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/include/asm/idtentry.h:697
RIP: 0010:smp_call_function_many_cond+0x811/0x15e0 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/smp.c:858
Code: 48 b9 00 00 00 00 00 fc ff df 48 8b 44 24 10 49 89 c6 83 e0 07 49 c1 ee 03 49 89 c4 49 01 ce 41 83 c4 03 e8 21 07 0c 00 f3 90 <41> 0f b6 06 41 38 c4 7c 08 84 c0 0f 85 6d 0b 00 00 44 8b 7b 08 31
RSP: 0018:ffffc90003c57880 EFLAGS: 00000293
RAX: 0000000000000000 RBX: ffff888062942680 RCX: ffffffff81ae5104
RDX: ffff888021ec3a00 RSI: ffffffff81ae50df RDI: 0000000000000005
RBP: 0000000000000002 R08: 0000000000000001 R09: 0000000000000001
R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000003
R13: ffff888135e3b740 R14: ffffed100c5284d1 R15: 0000000000000001
 on_each_cpu_cond_mask+0x40/0x90 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/smp.c:1043
 smp_text_poke_batch_finish+0x698/0xd50 home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/kernel/alternative.c:3002
 arch_jump_label_transform_apply+0x1c/0x30 home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/kernel/jump_label.c:145
 jump_label_update+0x369/0x540 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/jump_label.c:919
 static_key_disable_cpuslocked+0x15a/0x1c0 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/jump_label.c:240
 static_key_disable+0x1a/0x20 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/jump_label.c:248
 toggle_allocation_gate+0x145/0x250
 process_one_work+0x992/0x1b60 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/workqueue.c:3274
 process_scheduled_works home/wmy/Fuzzer/third_tool/linux-6.18/kernel/workqueue.c:3346 [inline]
 worker_thread+0x67e/0xe90 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/workqueue.c:3427
 kthread+0x3d0/0x780 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/kthread.c:463
 ret_from_fork+0x676/0x7d0 home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/kernel/process.c:195
 ret_from_fork_asm+0x1a/0x30 home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/entry/entry_64.S:245
 </TASK>
rcu: rcu_preempt kthread starved for 10485 jiffies! g357853 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=2
rcu: 	Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
rcu: RCU grace-period kthread stack dump:
task:rcu_preempt     state:R  running task     stack:28520 pid:16    tgid:16    ppid:2      task_flags:0x208040 flags:0x00080000
Call Trace:
 <TASK>
 sched_info_arrive home/wmy/Fuzzer/third_tool/linux-6.18/kernel/sched/stats.h:267 [inline]
 sched_info_switch home/wmy/Fuzzer/third_tool/linux-6.18/kernel/sched/stats.h:330 [inline]
 prepare_task_switch home/wmy/Fuzzer/third_tool/linux-6.18/kernel/sched/core.c:5122 [inline]
 context_switch home/wmy/Fuzzer/third_tool/linux-6.18/kernel/sched/core.c:5272 [inline]
 __schedule+0x1044/0x5bb0 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/sched/core.c:6929
 __schedule_loop home/wmy/Fuzzer/third_tool/linux-6.18/kernel/sched/core.c:7011 [inline]
 schedule+0xe7/0x3a0 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/sched/core.c:7026
 schedule_timeout+0x113/0x280 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/time/sleep_timeout.c:98
 rcu_gp_fqs_check_wake home/wmy/Fuzzer/third_tool/linux-6.18/kernel/rcu/tree.c:2007 [inline]
 rcu_gp_fqs_loop+0x18c/0xa00 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/rcu/tree.c:2083
 rcu_gp_kthread+0x26f/0x370 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/rcu/tree.c:2280
 kthread+0x3d0/0x780 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/kthread.c:463
 ret_from_fork+0x676/0x7d0 home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/kernel/process.c:195
 ret_from_fork_asm+0x1a/0x30 home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/entry/entry_64.S:245
 </TASK>
rcu: Stack dump where RCU GP kthread last ran:
CPU: 2 UID: 0 PID: 99214 Comm: syz-executor Not tainted 6.18.0 #1 PREEMPT(full) 
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
RIP: 0010:smp_call_function_many_cond+0x811/0x15e0 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/smp.c:858
Code: 48 b9 00 00 00 00 00 fc ff df 48 8b 44 24 10 49 89 c6 83 e0 07 49 c1 ee 03 49 89 c4 49 01 ce 41 83 c4 03 e8 21 07 0c 00 f3 90 <41> 0f b6 06 41 38 c4 7c 08 84 c0 0f 85 6d 0b 00 00 44 8b 7b 08 31
RSP: 0018:ffffc9000673f760 EFLAGS: 00000293
RAX: 0000000000000000 RBX: ffff888135f426a0 RCX: ffffffff81ae5104
RDX: ffff888024c39d00 RSI: ffffffff81ae50df RDI: 0000000000000005
RBP: 0000000000000003 R08: 0000000000000001 R09: 0000000000000001
R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000003
R13: ffff88806293b740 R14: ffffed1026be84d5 R15: 0000000000000001
FS:  0000555571e07500(0000) GS:ffff8880cf101000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f41e19556c0 CR3: 000000002aec0000 CR4: 00000000000006f0
Call Trace:
 <TASK>
 on_each_cpu_cond_mask+0x40/0x90 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/smp.c:1043
 consider_global_asid home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/mm/tlb.c:451 [inline]
 flush_tlb_mm_range+0x42d/0x17d0 home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/mm/tlb.c:1472
 dup_mmap+0xebb/0x20d0 home/wmy/Fuzzer/third_tool/linux-6.18/mm/mmap.c:1720
 futex_init_task home/wmy/Fuzzer/third_tool/linux-6.18/include/linux/futex.h:73 [inline]
 copy_process+0x36ba/0x76c0 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/fork.c:2230
 kernel_clone+0xea/0x8b0 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/fork.c:2609
 __do_sys_clone+0xce/0x120 home/wmy/Fuzzer/third_tool/linux-6.18/kernel/fork.c:2750
 do_syscall_64+0xcb/0xfa0 home/wmy/Fuzzer/third_tool/linux-6.18/arch/x86/entry/syscall_64.c:99
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f41e0ba6d07
Code: 00 00 90 f3 0f 1e fa 64 48 8b 04 25 10 00 00 00 45 31 c0 31 d2 31 f6 bf 11 00 20 01 4c 8d 90 d0 02 00 00 b8 38 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 41 41 89 c0 85 c0 75 2c 64 48 8b 04 25 10 00
RSP: 002b:00007ffdbbfd5598 EFLAGS: 00000246 ORIG_RAX: 0000000000000038
RAX: ffffffffffffffda RBX: 00007f41e195d680 RCX: 00007f41e0ba6d07
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000001200011
RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000001
R10: 0000555571e077d0 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000032 R14: 00007ffdbbfd5780 R15: 0000000000337977
 </TASK>


Best regards, 
Zhi Wang