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BUG: KCSAN: data-race in get_signal / ptrace_attach

write to 0xffff88815be2d070 of 4 bytes by task 24903 on cpu 0:
 ptrace_attach+0x243/0x510 kernel/ptrace.c:408
 __do_sys_ptrace kernel/ptrace.c:1281 [inline]
 __se_sys_ptrace+0xc1/0x1b0 kernel/ptrace.c:1261
 __x64_sys_ptrace+0x51/0x60 kernel/ptrace.c:1261
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88815be2d070 of 4 bytes by task 24902 on cpu 1:
 get_signal+0x733/0x1610 kernel/signal.c:2705
 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

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 24902 Comm: syz-executor.1 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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