bisecting fixing commit since 96d454cd2c1668010406ea4c28ab915bcbb747f4
building syzkaller on d5a3ae1f760e7cb2cd5a721d9645ae22eae114fe
testing commit 96d454cd2c1668010406ea4c28ab915bcbb747f4 with gcc (GCC) 8.1.0
kernel signature: f054bf0cdee851ff52bd0aec91237e298e82011da98459c9c7a8b5925f2d50f5
run #0: crashed: INFO: task hung in do_proc_bulk
run #1: crashed: INFO: task hung in do_proc_bulk
run #2: crashed: INFO: task hung in do_proc_bulk
run #3: crashed: INFO: task hung in do_proc_bulk
run #4: crashed: INFO: task hung in do_proc_bulk
run #5: crashed: INFO: task hung in do_proc_bulk
run #6: crashed: INFO: task hung in do_proc_bulk
run #7: crashed: INFO: task hung in do_proc_bulk
run #8: OK
run #9: OK
testing current HEAD ed8780e3f2ecc82645342d070c6b4e530532e680
testing commit ed8780e3f2ecc82645342d070c6b4e530532e680 with gcc (GCC) 8.1.0
kernel signature: 07c1a89ed95d4c570e3d1dc73b32e2803e1999e9e8dad3e966a567d92f3d09b8
run #0: crashed: INFO: task hung in do_proc_bulk
run #1: crashed: INFO: task hung in do_proc_bulk
run #2: crashed: INFO: task hung in do_proc_bulk
run #3: OK
run #4: OK
run #5: OK
run #6: OK
run #7: OK
run #8: OK
run #9: OK
revisions tested: 2, total time: 33m49.298931583s (build: 11m54.07024845s, test: 21m12.867090893s)
the crash still happens on HEAD
commit msg: Merge tag 'x86-urgent-2020-10-27' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
crash: INFO: task hung in do_proc_bulk
INFO: task syz-executor.0:8396 blocked for more than 143 seconds.
      Not tainted 5.10.0-rc1-syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz-executor.0  state:D stack:14088 pid: 8396 ppid:  6993 flags:0x00000004
Call Trace:
 context_switch kernel/sched/core.c:3774 [inline]
 __schedule+0x404/0x890 kernel/sched/core.c:4523
 schedule+0x38/0xe0 kernel/sched/core.c:4601
 schedule_timeout+0x233/0x2e0 kernel/time/timer.c:1852
 do_wait_for_common kernel/sched/completion.c:85 [inline]
 __wait_for_common kernel/sched/completion.c:106 [inline]
 wait_for_common kernel/sched/completion.c:117 [inline]
 wait_for_completion_timeout+0xa2/0x110 kernel/sched/completion.c:157
 usb_start_wait_urb+0x7d/0xf0 drivers/usb/core/message.c:63
 do_proc_bulk+0x13f/0x2f0 drivers/usb/core/devio.c:1231
 proc_bulk_compat drivers/usb/core/devio.c:2052 [inline]
 usbdev_do_ioctl drivers/usb/core/devio.c:2603 [inline]
 usbdev_ioctl+0x6e6/0x10e0 drivers/usb/core/devio.c:2708
 vfs_ioctl fs/ioctl.c:48 [inline]
 __do_sys_ioctl fs/ioctl.c:753 [inline]
 __se_sys_ioctl fs/ioctl.c:739 [inline]
 __x64_sys_ioctl+0x7c/0xb0 fs/ioctl.c:739
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x45d5b9
Code: Unable to access opcode bytes at RIP 0x45d58f.
RSP: 002b:00007f9c4c1b0c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 000000000001aa80 RCX: 000000000045d5b9
RDX: 0000000020000000 RSI: 00000000c0185502 RDI: 0000000000000003
RBP: 000000000118cf80 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 000000000118cf4c
R13: 00007ffd340d5c5f R14: 00007f9c4c1b19c0 R15: 000000000118cf4c

Showing all locks held in the system:
1 lock held by khungtaskd/1178:
 #0: ffffffff843d8940 (rcu_read_lock){....}-{1:2}, at: debug_show_all_locks+0x15/0x17a kernel/locking/lockdep.c:6260

=============================================

NMI backtrace for cpu 0
CPU: 0 PID: 1178 Comm: khungtaskd Not tainted 5.10.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0xa3/0xc8 lib/dump_stack.c:118
 nmi_cpu_backtrace.cold.8+0x53/0x6d lib/nmi_backtrace.c:105
 nmi_trigger_cpumask_backtrace+0xd5/0xf0 lib/nmi_backtrace.c:62
 trigger_all_cpu_backtrace include/linux/nmi.h:146 [inline]
 check_hung_uninterruptible_tasks kernel/hung_task.c:209 [inline]
 watchdog+0x57f/0x670 kernel/hung_task.c:295
 kthread+0x145/0x170 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1 skipped: idling at native_safe_halt+0xe/0x10 arch/x86/include/asm/irqflags.h:60