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BUG: KCSAN: data-race in prepare_signal / ptrace_attach

write to 0xffff8881e5b660b0 of 4 bytes by task 19992 on cpu 1:
 ptrace_attach+0x243/0x510 kernel/ptrace.c:407
 __do_sys_ptrace kernel/ptrace.c:1256 [inline]
 __se_sys_ptrace+0xc1/0x1b0 kernel/ptrace.c:1236
 __x64_sys_ptrace+0x51/0x60 kernel/ptrace.c:1236
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff8881e5b660b0 of 4 bytes by task 19991 on cpu 0:
 sig_ignored kernel/signal.c:115 [inline]
 prepare_signal+0x769/0xd80 kernel/signal.c:960
 __send_signal+0x64/0x760 kernel/signal.c:1080
 send_signal+0x38e/0x3d0 kernel/signal.c:1240
 do_send_sig_info kernel/signal.c:1289 [inline]
 do_send_specific+0x13d/0x1c0 kernel/signal.c:3825
 do_rt_tgsigqueueinfo kernel/signal.c:3945 [inline]
 __do_sys_rt_tgsigqueueinfo kernel/signal.c:3955 [inline]
 __se_sys_rt_tgsigqueueinfo+0x1f0/0x390 kernel/signal.c:3948
 __x64_sys_rt_tgsigqueueinfo+0x51/0x60 kernel/signal.c:3948
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 19991 Comm: syz-executor.2 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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