bisecting fixing commit since 7cc2a8ea104820dd9e702202621e8fd4d9f6c8cf
building syzkaller on 510951950dc0ee69cfdaf746061d3dbe31b49fd8
testing commit 7cc2a8ea104820dd9e702202621e8fd4d9f6c8cf with gcc (GCC) 8.1.0
kernel signature: 10a799ed06df693ceb058d3bb67fc280bc46b7c69c9fd3e2c75e4108830f26e3
run #0: crashed: INFO: rcu detected stall in tc_modify_qdisc
run #1: crashed: INFO: rcu detected stall in tc_modify_qdisc
run #2: crashed: INFO: rcu detected stall in tc_modify_qdisc
run #3: crashed: INFO: rcu detected stall in tc_modify_qdisc
run #4: crashed: INFO: rcu detected stall in tc_modify_qdisc
run #5: crashed: no output from test machine
run #6: crashed: no output from test machine
run #7: crashed: no output from test machine
run #8: crashed: no output from test machine
run #9: crashed: no output from test machine
testing current HEAD 6a9dc5fd6170d0a41c8a14eb19e63d94bea5705a
testing commit 6a9dc5fd6170d0a41c8a14eb19e63d94bea5705a with gcc (GCC) 8.1.0
kernel signature: 6ce0d9f8aae90d6df7147fad2c2da5c4f8d0ee549edae299ef05e9277d0e1232
run #0: crashed: INFO: rcu detected stall in tc_modify_qdisc
run #1: crashed: INFO: rcu detected stall in tc_modify_qdisc
run #2: crashed: INFO: rcu detected stall in tc_modify_qdisc
run #3: crashed: INFO: rcu detected stall in tc_modify_qdisc
run #4: crashed: INFO: rcu detected stall in tc_modify_qdisc
run #5: crashed: INFO: rcu detected stall in tc_modify_qdisc
run #6: crashed: no output from test machine
run #7: crashed: no output from test machine
run #8: crashed: no output from test machine
run #9: crashed: INFO: rcu detected stall in try_check_zero
revisions tested: 2, total time: 25m15.213315966s (build: 9m5.348275344s, test: 15m32.623520597s)
the crash still happens on HEAD
commit msg: lib: Revert use of fallthrough pseudo-keyword in lib/
crash: INFO: rcu detected stall in try_check_zero
rcu: INFO: rcu_preempt self-detected stall on CPU
rcu: 	1-...!: (1 GPs behind) idle=5d6/1/0x4000000000000000 softirq=8840/8844 fqs=1 
	(t=29745 jiffies g=3145 q=481)
NMI backtrace for cpu 1
CPU: 1 PID: 17 Comm: kworker/1:0 Not tainted 5.9.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: rcu_gp process_srcu
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0xb3/0xec lib/dump_stack.c:118
 nmi_cpu_backtrace.cold.8+0x3e/0x58 lib/nmi_backtrace.c:101
 nmi_trigger_cpumask_backtrace+0xd5/0xec lib/nmi_backtrace.c:62
 trigger_single_cpu_backtrace include/linux/nmi.h:164 [inline]
 rcu_dump_cpu_stacks+0xa2/0xce kernel/rcu/tree_stall.h:318
 print_cpu_stall kernel/rcu/tree_stall.h:551 [inline]
 check_cpu_stall kernel/rcu/tree_stall.h:625 [inline]
 rcu_pending kernel/rcu/tree.c:3637 [inline]
 rcu_sched_clock_irq.cold.94+0x61/0x52d kernel/rcu/tree.c:2519
 update_process_times+0x1f/0x60 kernel/time/timer.c:1710
 tick_sched_handle.isra.23+0x1a/0x50 kernel/time/tick-sched.c:176
 tick_sched_timer+0x6c/0x80 kernel/time/tick-sched.c:1328
 __run_hrtimer kernel/time/hrtimer.c:1524 [inline]
 __hrtimer_run_queues+0x1e1/0x510 kernel/time/hrtimer.c:1588
 hrtimer_interrupt+0xf9/0x210 kernel/time/hrtimer.c:1650
 local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1080 [inline]
 __sysvec_apic_timer_interrupt+0x8e/0x2b0 arch/x86/kernel/apic/apic.c:1097
 asm_call_on_stack+0xf/0x20 arch/x86/entry/entry_64.S:706
 </IRQ>
 __run_on_irqstack arch/x86/include/asm/irq_stack.h:22 [inline]
 run_on_irqstack_cond arch/x86/include/asm/irq_stack.h:48 [inline]
 sysvec_apic_timer_interrupt+0xc1/0xe0 arch/x86/kernel/apic/apic.c:1091
 asm_sysvec_apic_timer_interrupt+0x12/0x20 arch/x86/include/asm/idtentry.h:581
RIP: 0010:preempt_count_add+0x7/0x90 kernel/sched/core.c:4174
Code: 44 00 00 48 83 3d 48 9e 0c 03 00 74 08 9c 58 0f 1f 44 00 00 c3 0f 0b 66 90 66 2e 0f 1f 84 00 00 00 00 00 8b 15 62 50 b2 04 53 <89> fb 65 01 3d c0 7e e1 7e 85 d2 75 11 65 8b 05 b5 7e e1 7e 0f b6
RSP: 0018:ffffc90000cfbd68 EFLAGS: 00000286
RAX: 0000000080000000 RBX: 000000b7efd783e1 RCX: 0000000000000001
RDX: 0000000000000000 RSI: ffffffff84015989 RDI: 0000000000000001
RBP: 000000b7efd76c86 R08: 0000000000000000 R09: 0000000000000000
R10: ffffc90000cfbdd8 R11: 548d577855406d15 R12: 0000000000002ced
R13: 0000000000000001 R14: 000000000000156d R15: 00000000ffffffff
 delay_tsc+0x38/0xb0 arch/x86/lib/delay.c:79
 try_check_zero+0xbf/0xf0 kernel/rcu/srcutree.c:718
 srcu_advance_state kernel/rcu/srcutree.c:1158 [inline]
 process_srcu+0x160/0x4d0 kernel/rcu/srcutree.c:1253
 process_one_work+0x26a/0x5f0 kernel/workqueue.c:2269
 worker_thread+0x38/0x380 kernel/workqueue.c:2415
 kthread+0x148/0x170 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294