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BUG: KCSAN: data-race in __ptrace_unlink / ptrace_stop

read to 0xffff88807a593064 of 4 bytes by task 16991 on cpu 0:
 __ptrace_unlink+0x17d/0x380 kernel/ptrace.c:144
 __ptrace_detach kernel/ptrace.c:523 [inline]
 exit_ptrace+0xb4/0x200 kernel/ptrace.c:583
 forget_original_parent kernel/exit.c:616 [inline]
 exit_notify kernel/exit.c:656 [inline]
 do_exit+0x14d6/0x18c0 kernel/exit.c:818
 do_group_exit+0xb4/0x1c0 kernel/exit.c:895
 get_signal+0x2a2/0x1320 kernel/signal.c:2734
 do_signal+0x2f/0x6c0 arch/x86/kernel/signal.c:815
 exit_to_usermode_loop+0x250/0x2c0 arch/x86/entry/common.c:160
 prepare_exit_to_usermode arch/x86/entry/common.c:195 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:278 [inline]
 do_syscall_64+0x384/0x3a0 arch/x86/entry/common.c:304
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff88807a593064 of 4 bytes by task 16921 on cpu 1:
 freezer_count include/linux/freezer.h:121 [inline]
 freezable_schedule include/linux/freezer.h:173 [inline]
 ptrace_stop+0x23f/0x520 kernel/signal.c:2210
 ptrace_signal kernel/signal.c:2485 [inline]
 get_signal+0xc69/0x1320 kernel/signal.c:2630
 do_signal+0x2f/0x6c0 arch/x86/kernel/signal.c:815
 exit_to_usermode_loop+0x250/0x2c0 arch/x86/entry/common.c:160
 prepare_exit_to_usermode arch/x86/entry/common.c:195 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:278 [inline]
 do_syscall_64+0x384/0x3a0 arch/x86/entry/common.c:304
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 16921 Comm: syz-executor.4 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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