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BUG: KCSAN: data-race in __ptrace_unlink / do_exit

write to 0xffff88811dc1c028 of 4 bytes by task 32671 on cpu 1:
 __ptrace_unlink+0x160/0x300 kernel/ptrace.c:132
 __ptrace_detach+0x1b/0x1a0 kernel/ptrace.c:528
 exit_ptrace+0x93/0x160 kernel/ptrace.c:588
 forget_original_parent kernel/exit.c:610 [inline]
 exit_notify kernel/exit.c:650 [inline]
 do_exit+0x13d1/0x16c0 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:136 [inline]
 exit_to_user_mode_prepare+0x124/0x210 kernel/entry/common.c:167
 syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:242
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88811dc1c028 of 4 bytes by task 32708 on cpu 0:
 ptrace_event_enabled include/linux/ptrace.h:142 [inline]
 ptrace_event include/linux/ptrace.h:157 [inline]
 do_exit+0x13c/0x16c0 kernel/exit.c:747
 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:136 [inline]
 exit_to_user_mode_prepare+0x124/0x210 kernel/entry/common.c:167
 syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:242
 ret_from_fork+0x15/0x30 arch/x86/entry/entry_64.S:287

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 32708 Comm: syz-executor.0 Not tainted 5.9.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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