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BUG: KCSAN: data-race in detach_if_pending / timer_reduce

read to 0xffff88809b167d18 of 8 bytes by task 7721 on cpu 0:
 __mod_timer kernel/time/timer.c:963 [inline]
 timer_reduce+0x5d/0x830 kernel/time/timer.c:1115
 cgroup_file_notify+0x11f/0x130 kernel/cgroup/cgroup.c:4097
 pids_can_fork+0x176/0x1b0 kernel/cgroup/pids.c:233
 cgroup_can_fork+0xde/0x1b0 kernel/cgroup/cgroup.c:5881
 copy_process+0x263b/0x3c40 kernel/fork.c:2175
 _do_fork+0xfe/0x7a0 kernel/fork.c:2421
 __do_sys_clone kernel/fork.c:2576 [inline]
 __se_sys_clone kernel/fork.c:2557 [inline]
 __x64_sys_clone+0x130/0x170 kernel/fork.c:2557
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff88809b167d18 of 8 bytes by task 10 on cpu 1:
 __hlist_del include/linux/list.h:764 [inline]
 detach_timer kernel/time/timer.c:815 [inline]
 detach_if_pending+0xcd/0x290 kernel/time/timer.c:832
 try_to_del_timer_sync+0x60/0xb0 kernel/time/timer.c:1226
 del_timer_sync+0x6b/0xa0 kernel/time/timer.c:1365
 schedule_timeout+0x2d2/0x6e0 kernel/time/timer.c:1896
 rcu_gp_fqs_loop+0x37c/0x580 kernel/rcu/tree.c:1661
 rcu_gp_kthread+0x143/0x230 kernel/rcu/tree.c:1821
 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 10 Comm: rcu_preempt Not tainted 5.5.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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