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BUG: KCSAN: data-race in __ptrace_unlink / do_exit

write to 0xffff888178d970b0 of 4 bytes by task 11676 on cpu 0:
 __ptrace_unlink+0x162/0x320 kernel/ptrace.c:133
 __ptrace_detach+0x1b/0x1d0 kernel/ptrace.c:523
 exit_ptrace+0x93/0x160 kernel/ptrace.c:583
 forget_original_parent kernel/exit.c:629 [inline]
 exit_notify kernel/exit.c:669 [inline]
 do_exit+0x126d/0x1560 kernel/exit.c:846
 do_group_exit+0xce/0x1a0 kernel/exit.c:923
 get_signal+0xfc3/0x1610 kernel/signal.c:2818
 arch_do_signal_or_restart+0x2a/0x220 arch/x86/kernel/signal.c:789
 handle_signal_work kernel/entry/common.c:147 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:171 [inline]
 exit_to_user_mode_prepare+0x109/0x190 kernel/entry/common.c:208
 __syscall_exit_to_user_mode_work kernel/entry/common.c:290 [inline]
 syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:301
 do_syscall_64+0x56/0x90 arch/x86/entry/common.c:57
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff888178d970b0 of 4 bytes by task 11679 on cpu 1:
 ptrace_event_enabled include/linux/ptrace.h:142 [inline]
 ptrace_event include/linux/ptrace.h:157 [inline]
 do_exit+0x101/0x1560 kernel/exit.c:766
 do_group_exit+0xce/0x1a0 kernel/exit.c:923
 get_signal+0xfc3/0x1610 kernel/signal.c:2818
 arch_do_signal_or_restart+0x2a/0x220 arch/x86/kernel/signal.c:789
 handle_signal_work kernel/entry/common.c:147 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:171 [inline]
 exit_to_user_mode_prepare+0x109/0x190 kernel/entry/common.c:208
 __syscall_exit_to_user_mode_work kernel/entry/common.c:290 [inline]
 syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:301
 ret_from_fork+0x15/0x30 arch/x86/entry/entry_64.S:287

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 11679 Comm: syz-executor.3 Not tainted 5.12.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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