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SubjectWARNING: suspicious RCU usage in ext4_release_system_zone

syzbot found the following crash on:

HEAD commit: ed858b88 Add linux-next specific files for 20190826
git tree: linux-next
console output:
kernel config:
dashboard link:
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:

WARNING: suspicious RCU usage
5.3.0-rc6-next-20190826 #73 Not tainted
fs/ext4/block_validity.c:333 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
1 lock held by syz-executor.4/8779:
#0: ffff888091c4a518 (&type->s_umount_key#32){++++}, at: deactivate_super
fs/super.c:363 [inline]
#0: ffff888091c4a518 (&type->s_umount_key#32){++++}, at:
deactivate_super+0x1aa/0x1d0 fs/super.c:360

stack backtrace:
CPU: 0 PID: 8779 Comm: syz-executor.4 Not tainted 5.3.0-rc6-next-20190826
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+0x172/0x1f0 lib/dump_stack.c:113
lockdep_rcu_suspicious+0x153/0x15d kernel/locking/lockdep.c:5438
ext4_release_system_zone+0x166/0x1a0 fs/ext4/block_validity.c:333
ext4_put_super+0x954/0xd70 fs/ext4/super.c:992
generic_shutdown_super+0x14c/0x370 fs/super.c:460
kill_block_super+0xa0/0x100 fs/super.c:1434
deactivate_locked_super+0x95/0x100 fs/super.c:333
deactivate_super fs/super.c:364 [inline]
deactivate_super+0x1b2/0x1d0 fs/super.c:360
cleanup_mnt+0x351/0x4c0 fs/namespace.c:1102
__cleanup_mnt+0x16/0x20 fs/namespace.c:1109
task_work_run+0x145/0x1c0 kernel/task_work.c:113
tracehook_notify_resume include/linux/tracehook.h:188 [inline]
exit_to_usermode_loop+0x316/0x380 arch/x86/entry/common.c:163
prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
syscall_return_slowpath arch/x86/entry/common.c:274 [inline]
do_syscall_64+0x65f/0x760 arch/x86/entry/common.c:300
RIP: 0033:0x45c2a7
Code: 64 89 04 25 d0 02 00 00 58 5f ff d0 48 89 c7 e8 2f be ff ff 66 2e 0f
1f 84 00 00 00 00 00 0f 1f 44 00 00 b8 a6 00 00 00 0f 05 <48> 3d 01 f0 ff
ff 0f 83 9d 8d fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fffdccad768 EFLAGS: 00000202 ORIG_RAX: 00000000000000a6
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 000000000045c2a7
RDX: 0000000000403520 RSI: 0000000000000002 RDI: 00007fffdccad810
RBP: 000000000000011b R08: 0000000000000000 R09: 000000000000000b
R10: 0000000000000005 R11: 0000000000000202 R12: 00007fffdccae8a0
R13: 00005555565a4940 R14: 0000000000000000 R15: 00007fffdccae8a0
------------[ cut here ]------------
ODEBUG: free active (active state 0) object type: percpu_counter hint: 0x0
WARNING: CPU: 1 PID: 8779 at lib/debugobjects.c:481
debug_print_object+0x168/0x250 lib/debugobjects.c:481

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