possible deadlock in blocking_notifier_call_chain

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: a844dc4c Linux 4.14.158
git tree: linux-4.14.y
console output: https://syzkaller.appspot.com/x/log.txt?x=14827646e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=c02bef505ffc02ff
dashboard link: https://syzkaller.appspot.com/bug?extid=347efd5cd55d8ca41479
compiler: gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+347efd...@syzkaller.appspotmail.com

kobject: 'loop0' (ffff8880a1a76ee0): kobject_uevent_env
kobject: 'loop0' (ffff8880a1a76ee0): fill_kobj_path: path
= '/devices/virtual/block/loop0'
============================================
WARNING: possible recursive locking detected
4.14.158-syzkaller #0 Not tainted
--------------------------------------------
syz-executor.3/2792 is trying to acquire lock:
((fb_notifier_list).rwsem){++++}, at: [<ffffffff813ee86a>]
__blocking_notifier_call_chain kernel/notifier.c:316 [inline]
((fb_notifier_list).rwsem){++++}, at: [<ffffffff813ee86a>]
__blocking_notifier_call_chain kernel/notifier.c:304 [inline]
((fb_notifier_list).rwsem){++++}, at: [<ffffffff813ee86a>]
blocking_notifier_call_chain kernel/notifier.c:328 [inline]
((fb_notifier_list).rwsem){++++}, at: [<ffffffff813ee86a>]
blocking_notifier_call_chain+0x6a/0xa0 kernel/notifier.c:325

but task is already holding lock:
((fb_notifier_list).rwsem){++++}, at: [<ffffffff813ee86a>]
__blocking_notifier_call_chain kernel/notifier.c:316 [inline]
((fb_notifier_list).rwsem){++++}, at: [<ffffffff813ee86a>]
__blocking_notifier_call_chain kernel/notifier.c:304 [inline]
((fb_notifier_list).rwsem){++++}, at: [<ffffffff813ee86a>]
blocking_notifier_call_chain kernel/notifier.c:328 [inline]
((fb_notifier_list).rwsem){++++}, at: [<ffffffff813ee86a>]
blocking_notifier_call_chain+0x6a/0xa0 kernel/notifier.c:325

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock((fb_notifier_list).rwsem);
lock((fb_notifier_list).rwsem);

*** DEADLOCK ***

May be due to missing lock nesting notation

3 locks held by syz-executor.3/2792:
#0: (console_lock){+.+.}, at: [<ffffffff832479e3>]
do_fb_ioctl+0x143/0x940 drivers/video/fbdev/core/fbmem.c:1212
#1: (&fb_info->lock){+.+.}, at: [<ffffffff83245b6f>]
lock_fb_info+0x1f/0x80 drivers/video/fbdev/core/fbmem.c:81
#2: ((fb_notifier_list).rwsem){++++}, at: [<ffffffff813ee86a>]
__blocking_notifier_call_chain kernel/notifier.c:316 [inline]
#2: ((fb_notifier_list).rwsem){++++}, at: [<ffffffff813ee86a>]
__blocking_notifier_call_chain kernel/notifier.c:304 [inline]
#2: ((fb_notifier_list).rwsem){++++}, at: [<ffffffff813ee86a>]
blocking_notifier_call_chain kernel/notifier.c:328 [inline]
#2: ((fb_notifier_list).rwsem){++++}, at: [<ffffffff813ee86a>]
blocking_notifier_call_chain+0x6a/0xa0 kernel/notifier.c:325

stack backtrace:
CPU: 1 PID: 2792 Comm: syz-executor.3 Not tainted 4.14.158-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0x142/0x197 lib/dump_stack.c:58
print_deadlock_bug kernel/locking/lockdep.c:1796 [inline]
check_deadlock kernel/locking/lockdep.c:1843 [inline]
validate_chain kernel/locking/lockdep.c:2444 [inline]
__lock_acquire.cold+0x2bf/0x8dc kernel/locking/lockdep.c:3487
lock_acquire+0x16f/0x430 kernel/locking/lockdep.c:3994
down_read+0x3b/0xb0 kernel/locking/rwsem.c:24
__blocking_notifier_call_chain kernel/notifier.c:316 [inline]
__blocking_notifier_call_chain kernel/notifier.c:304 [inline]
blocking_notifier_call_chain kernel/notifier.c:328 [inline]
blocking_notifier_call_chain+0x6a/0xa0 kernel/notifier.c:325
fb_notifier_call_chain+0x25/0x30 drivers/video/fbdev/core/fb_notify.c:45
fb_set_var+0xb09/0xcf0 drivers/video/fbdev/core/fbmem.c:1054
fbcon_switch+0x4f6/0x1820 drivers/video/fbdev/core/fbcon.c:2232
redraw_screen+0x335/0x7c0 drivers/tty/vt/vt.c:688
fbcon_blank+0xa04/0xde0 drivers/video/fbdev/core/fbcon.c:2370
do_unblank_screen drivers/tty/vt/vt.c:3909 [inline]
do_unblank_screen+0x207/0x500 drivers/tty/vt/vt.c:3876
fbcon_fb_blanked drivers/video/fbdev/core/fbcon.c:3211 [inline]
fbcon_event_notify+0x1499/0x17af drivers/video/fbdev/core/fbcon.c:3329
notifier_call_chain+0x111/0x1b0 kernel/notifier.c:93
__blocking_notifier_call_chain kernel/notifier.c:317 [inline]
__blocking_notifier_call_chain kernel/notifier.c:304 [inline]
blocking_notifier_call_chain kernel/notifier.c:328 [inline]
blocking_notifier_call_chain+0x80/0xa0 kernel/notifier.c:325
fb_notifier_call_chain+0x25/0x30 drivers/video/fbdev/core/fb_notify.c:45
fb_blank+0x162/0x1b0 drivers/video/fbdev/core/fbmem.c:1082
do_fb_ioctl+0x196/0x940 drivers/video/fbdev/core/fbmem.c:1218
fb_ioctl+0xe6/0x130 drivers/video/fbdev/core/fbmem.c:1242
vfs_ioctl fs/ioctl.c:46 [inline]
file_ioctl fs/ioctl.c:500 [inline]
do_vfs_ioctl+0x7ae/0x1060 fs/ioctl.c:684
SYSC_ioctl fs/ioctl.c:701 [inline]
SyS_ioctl+0x8f/0xc0 fs/ioctl.c:692
do_syscall_64+0x1e8/0x640 arch/x86/entry/common.c:292
entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x45a909
RSP: 002b:00007f6d1cbdac78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 000000000045a909
RDX: 0000000000000000 RSI: 0000000000004611 RDI: 0000000000000003
RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f6d1cbdb6d4
R13: 00000000004c305f R14: 00000000004d8318 R15: 00000000ffffffff
kobject: 'loop0' (ffff8880a1a76ee0): kobject_uevent_env
kobject: 'loop0' (ffff8880a1a76ee0): fill_kobj_path: path
= '/devices/virtual/block/loop0'
kobject: 'loop0' (ffff8880a1a76ee0): kobject_uevent_env
kobject: 'loop0' (ffff8880a1a76ee0): fill_kobj_path: path
= '/devices/virtual/block/loop0'
kobject: 'loop4' (ffff8880a417cee0): kobject_uevent_env
kobject: 'loop4' (ffff8880a417cee0): fill_kobj_path: path
= '/devices/virtual/block/loop4'
kobject: 'loop5' (ffff8880a41bb7e0): kobject_uevent_env
kobject: 'loop5' (ffff8880a41bb7e0): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop0' (ffff8880a1a76ee0): kobject_uevent_env
kobject: 'loop0' (ffff8880a1a76ee0): fill_kobj_path: path
= '/devices/virtual/block/loop0'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
ntfs: (device loop3): parse_options(): Unrecognized mount option .
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
ntfs: (device loop3): parse_options(): Unrecognized mount option .
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
ntfs: (device loop3): parse_options(): Unrecognized mount option .
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
ntfs: (device loop3): parse_options(): Unrecognized mount option .
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop4' (ffff8880a417cee0): kobject_uevent_env
kobject: 'loop4' (ffff8880a417cee0): fill_kobj_path: path
= '/devices/virtual/block/loop4'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop5' (ffff8880a41bb7e0): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop5' (ffff8880a41bb7e0): fill_kobj_path: path
= '/devices/virtual/block/loop5'
ntfs: (device loop3): parse_options(): Unrecognized mount option .
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop0' (ffff8880a1a76ee0): kobject_uevent_env
kobject: 'loop0' (ffff8880a1a76ee0): fill_kobj_path: path
= '/devices/virtual/block/loop0'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop5' (ffff8880a41bb7e0): kobject_uevent_env
kobject: 'loop5' (ffff8880a41bb7e0): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop4' (ffff8880a417cee0): kobject_uevent_env
kobject: 'loop4' (ffff8880a417cee0): fill_kobj_path: path
= '/devices/virtual/block/loop4'
kobject: 'loop5' (ffff8880a41bb7e0): kobject_uevent_env
kobject: 'loop5' (ffff8880a41bb7e0): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop0' (ffff8880a1a76ee0): kobject_uevent_env
kobject: 'loop0' (ffff8880a1a76ee0): fill_kobj_path: path
= '/devices/virtual/block/loop0'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop5' (ffff8880a41bb7e0): kobject_uevent_env
kobject: 'loop5' (ffff8880a41bb7e0): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop5' (ffff8880a41bb7e0): kobject_uevent_env
kobject: 'loop5' (ffff8880a41bb7e0): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop4' (ffff8880a417cee0): kobject_uevent_env
kobject: 'loop4' (ffff8880a417cee0): fill_kobj_path: path
= '/devices/virtual/block/loop4'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop0' (ffff8880a1a76ee0): kobject_uevent_env
kobject: 'loop0' (ffff8880a1a76ee0): fill_kobj_path: path
= '/devices/virtual/block/loop0'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop4' (ffff8880a417cee0): kobject_uevent_env
kobject: 'loop4' (ffff8880a417cee0): fill_kobj_path: path
= '/devices/virtual/block/loop4'
kobject: 'loop5' (ffff8880a41bb7e0): kobject_uevent_env
kobject: 'loop5' (ffff8880a41bb7e0): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop0' (ffff8880a1a76ee0): kobject_uevent_env
kobject: 'loop0' (ffff8880a1a76ee0): fill_kobj_path: path
= '/devices/virtual/block/loop0'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop4' (ffff8880a417cee0): kobject_uevent_env
kobject: 'loop4' (ffff8880a417cee0): fill_kobj_path: path
= '/devices/virtual/block/loop4'
kobject: 'loop5' (ffff8880a41bb7e0): kobject_uevent_env
kobject: 'loop5' (ffff8880a41bb7e0): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop0' (ffff8880a1a76ee0): kobject_uevent_env
kobject: 'loop0' (ffff8880a1a76ee0): fill_kobj_path: path
= '/devices/virtual/block/loop0'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop4' (ffff8880a417cee0): kobject_uevent_env
kobject: 'loop4' (ffff8880a417cee0): fill_kobj_path: path
= '/devices/virtual/block/loop4'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop5' (ffff8880a41bb7e0): kobject_uevent_env
kobject: 'loop5' (ffff8880a41bb7e0): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
net_ratelimit: 10 callbacks suppressed
protocol 88fb is buggy, dev hsr_slave_0
protocol 88fb is buggy, dev hsr_slave_1
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop5' (ffff8880a41bb7e0): kobject_uevent_env
kobject: 'loop5' (ffff8880a41bb7e0): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
protocol 88fb is buggy, dev hsr_slave_0
protocol 88fb is buggy, dev hsr_slave_1
kobject: 'loop5' (ffff8880a41bb7e0): kobject_uevent_env
kobject: 'loop5' (ffff8880a41bb7e0): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop4' (ffff8880a417cee0): kobject_uevent_env
kobject: 'loop4' (ffff8880a417cee0): fill_kobj_path: path
= '/devices/virtual/block/loop4'
kobject: 'loop0' (ffff8880a1a76ee0): kobject_uevent_env
kobject: 'loop0' (ffff8880a1a76ee0): fill_kobj_path: path
= '/devices/virtual/block/loop0'
protocol 88fb is buggy, dev hsr_slave_0
protocol 88fb is buggy, dev hsr_slave_1
kobject: 'loop5' (ffff8880a41bb7e0): kobject_uevent_env
kobject: 'loop5' (ffff8880a41bb7e0): fill_kobj_path: path
= '/devices/virtual/block/loop5'
kobject: 'loop1' (ffff8880a407c4e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a407c4e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop3' (ffff8880a4142e60): kobject_uevent_env
kobject: 'loop3' (ffff8880a4142e60): fill_kobj_path: path
= '/devices/virtual/block/loop3'
kobject: 'loop4' (ffff8880a417cee0): kobject_uevent_env
kobject: 'loop4' (ffff8880a417cee0): fill_kobj_path: path
= '/devices/virtual/block/loop4'
kobject: 'loop4' (ffff8880a417cee0): kobject_uevent_env
kobject: 'loop4' (ffff8880a417cee0): fill_kobj_path: path
= '/devices/virtual/block/loop4'
kobject: 'loop4' (ffff8880a417cee0): kobject_uevent_env
kobject: 'loop4' (ffff8880a417cee0): fill_kobj_path: path
= '/devices/virtual/block/loop4'


---
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[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit: c10b57a5 Linux 4.14.176
git tree: linux-4.14.y
console output: https://syzkaller.appspot.com/x/log.txt?x=126897b3e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=457897b554d08537

dashboard link: https://syzkaller.appspot.com/bug?extid=347efd5cd55d8ca41479
compiler: gcc (GCC) 9.0.0 20181231 (experimental)

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=15e2ba20100000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=169fc93be00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+347efd...@syzkaller.appspotmail.com

============================================
WARNING: possible recursive locking detected

4.14.176-syzkaller #0 Not tainted
--------------------------------------------
syz-executor013/6330 is trying to acquire lock:
((fb_notifier_list).rwsem){++++}, at: [<ffffffff813cdc73>] __blocking_notifier_call_chain kernel/notifier.c:316 [inline]
((fb_notifier_list).rwsem){++++}, at: [<ffffffff813cdc73>] __blocking_notifier_call_chain kernel/notifier.c:304 [inline]
((fb_notifier_list).rwsem){++++}, at: [<ffffffff813cdc73>] blocking_notifier_call_chain kernel/notifier.c:328 [inline]
((fb_notifier_list).rwsem){++++}, at: [<ffffffff813cdc73>] blocking_notifier_call_chain+0x63/0x90 kernel/notifier.c:325

but task is already holding lock:

((fb_notifier_list).rwsem){++++}, at: [<ffffffff813cdc73>] __blocking_notifier_call_chain kernel/notifier.c:316 [inline]
((fb_notifier_list).rwsem){++++}, at: [<ffffffff813cdc73>] __blocking_notifier_call_chain kernel/notifier.c:304 [inline]
((fb_notifier_list).rwsem){++++}, at: [<ffffffff813cdc73>] blocking_notifier_call_chain kernel/notifier.c:328 [inline]
((fb_notifier_list).rwsem){++++}, at: [<ffffffff813cdc73>] blocking_notifier_call_chain+0x63/0x90 kernel/notifier.c:325

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock((fb_notifier_list).rwsem);
lock((fb_notifier_list).rwsem);

*** DEADLOCK ***

May be due to missing lock nesting notation

3 locks held by syz-executor013/6330:
#0: (console_lock){+.+.}, at: [<ffffffff83170143>] do_fb_ioctl+0x143/0x940 drivers/video/fbdev/core/fbmem.c:1212
#1: (&fb_info->lock){+.+.}, at: [<ffffffff8316e39a>] lock_fb_info+0x1a/0x70 drivers/video/fbdev/core/fbmem.c:81
#2: ((fb_notifier_list).rwsem){++++}, at: [<ffffffff813cdc73>] __blocking_notifier_call_chain kernel/notifier.c:316 [inline]
#2: ((fb_notifier_list).rwsem){++++}, at: [<ffffffff813cdc73>] __blocking_notifier_call_chain kernel/notifier.c:304 [inline]
#2: ((fb_notifier_list).rwsem){++++}, at: [<ffffffff813cdc73>] blocking_notifier_call_chain kernel/notifier.c:328 [inline]
#2: ((fb_notifier_list).rwsem){++++}, at: [<ffffffff813cdc73>] blocking_notifier_call_chain+0x63/0x90 kernel/notifier.c:325

stack backtrace:
CPU: 1 PID: 6330 Comm: syz-executor013 Not tainted 4.14.176-syzkaller #0

Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]

dump_stack+0x13e/0x194 lib/dump_stack.c:58

print_deadlock_bug kernel/locking/lockdep.c:1796 [inline]
check_deadlock kernel/locking/lockdep.c:1843 [inline]
validate_chain kernel/locking/lockdep.c:2444 [inline]
__lock_acquire.cold+0x2bf/0x8dc kernel/locking/lockdep.c:3487

lock_acquire+0x170/0x3f0 kernel/locking/lockdep.c:3994
down_read+0x37/0xa0 kernel/locking/rwsem.c:24

__blocking_notifier_call_chain kernel/notifier.c:316 [inline]
__blocking_notifier_call_chain kernel/notifier.c:304 [inline]
blocking_notifier_call_chain kernel/notifier.c:328 [inline]

blocking_notifier_call_chain+0x63/0x90 kernel/notifier.c:325
fb_set_var+0xaad/0xc70 drivers/video/fbdev/core/fbmem.c:1054
fbcon_switch+0x4b9/0x1780 drivers/video/fbdev/core/fbcon.c:2235
redraw_screen+0x331/0x770 drivers/tty/vt/vt.c:689
fbcon_blank+0x9b3/0xd60 drivers/video/fbdev/core/fbcon.c:2373
do_unblank_screen drivers/tty/vt/vt.c:3929 [inline]
do_unblank_screen+0x1fd/0x4f0 drivers/tty/vt/vt.c:3896
fbcon_fb_blanked drivers/video/fbdev/core/fbcon.c:3214 [inline]
fbcon_event_notify+0x1433/0x1746 drivers/video/fbdev/core/fbcon.c:3332
notifier_call_chain+0x107/0x1a0 kernel/notifier.c:93

__blocking_notifier_call_chain kernel/notifier.c:317 [inline]
__blocking_notifier_call_chain kernel/notifier.c:304 [inline]
blocking_notifier_call_chain kernel/notifier.c:328 [inline]

blocking_notifier_call_chain+0x79/0x90 kernel/notifier.c:325
fb_blank+0x14c/0x190 drivers/video/fbdev/core/fbmem.c:1082
do_fb_ioctl+0x196/0x940 drivers/video/fbdev/core/fbmem.c:1218
fb_ioctl+0xdd/0x130 drivers/video/fbdev/core/fbmem.c:1242

vfs_ioctl fs/ioctl.c:46 [inline]
file_ioctl fs/ioctl.c:500 [inline]

do_vfs_ioctl+0x75a/0xfe0 fs/ioctl.c:684
SYSC_ioctl fs/ioctl.c:701 [inline]
SyS_ioctl+0x7f/0xb0 fs/ioctl.c:692
do_syscall_64+0x1d5/0x640 arch/x86/entry/common.c:292
entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x4413c9
RSP: 002b:00007ffc5a2a0d38 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00000000004413c9
RDX: 0000000000000000 RSI: 0000000000004611 RDI: 0000000000000004
RBP: 0000000000008ac3 R08: 00000000004002c8 R09: 00000000004002c8
R10: 00000000004002c8 R11: 0000000000000246 R12: 00000000004021f0
R13: 0000000000402280 R14: 0000000000000000 R15: 0000000000000000