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BUG: KCSAN: data-race in prepare_signal / ptrace_attach

write to 0xffff88813c2170f0 of 4 bytes by task 18179 on cpu 1:
 ptrace_attach+0x243/0x510 kernel/ptrace.c:424
 __do_sys_ptrace kernel/ptrace.c:1297 [inline]
 __se_sys_ptrace+0xc1/0x1b0 kernel/ptrace.c:1277
 __x64_sys_ptrace+0x51/0x60 kernel/ptrace.c:1277
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88813c2170f0 of 4 bytes by task 18180 on cpu 0:
 sig_ignored kernel/signal.c:115 [inline]
 prepare_signal+0xbdd/0xdd0 kernel/signal.c:962
 __send_signal+0x64/0x760 kernel/signal.c:1082
 send_signal+0x281/0x390 kernel/signal.c:1245
 do_send_sig_info kernel/signal.c:1294 [inline]
 do_send_specific+0x13d/0x1c0 kernel/signal.c:3842
 do_tkill kernel/signal.c:3868 [inline]
 __do_sys_tkill kernel/signal.c:3903 [inline]
 __se_sys_tkill kernel/signal.c:3897 [inline]
 __x64_sys_tkill+0xe2/0x120 kernel/signal.c:3897
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x00000000 -> 0x00010001

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 18180 Comm: syz-executor.0 Not tainted 5.13.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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