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BUG: KCSAN: data-race in acct_collect / do_exit

write to 0xffff8880949244bc of 4 bytes by task 861 on cpu 1:
 reparent_leader kernel/exit.c:582 [inline]
 forget_original_parent kernel/exit.c:634 [inline]
 exit_notify kernel/exit.c:650 [inline]
 do_exit+0xbae/0x16c0 kernel/exit.c:826
 do_group_exit+0xcb/0x180 kernel/exit.c:903
 get_signal+0xe9f/0x1650 kernel/signal.c:2757
 arch_do_signal+0x25/0x270 arch/x86/kernel/signal.c:811
 exit_to_user_mode_loop kernel/entry/common.c:161 [inline]
 exit_to_user_mode_prepare+0x124/0x210 kernel/entry/common.c:192
 syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:267
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880949244bc of 4 bytes by task 866 on cpu 0:
 thread_group_leader include/linux/sched/signal.h:654 [inline]
 acct_collect+0x149/0x3a0 kernel/acct.c:556
 do_exit+0x3ba/0x16c0 kernel/exit.c:785
 do_group_exit+0xa8/0x180 kernel/exit.c:903
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:914
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:912
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:912
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 866 Comm: syz-executor.2 Not tainted 5.9.0-rc8-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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