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BUG: KCSAN: data-race in ptrace_check_attach / zap_other_threads

write to 0xffff88809cc7f488 of 8 bytes by task 13227 on cpu 0:
 task_clear_jobctl_pending kernel/signal.c:344 [inline]
 zap_other_threads+0x98/0x1e0 kernel/signal.c:1355
 do_group_exit+0x146/0x180 kernel/exit.c:898
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:914
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:912
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:912
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88809cc7f488 of 8 bytes by task 13229 on cpu 1:
 ptrace_freeze_traced kernel/ptrace.c:178 [inline]
 ptrace_check_attach+0xd8/0x220 kernel/ptrace.c:247
 __do_sys_ptrace kernel/ptrace.c:1272 [inline]
 __se_sys_ptrace+0xdb/0x2d0 kernel/ptrace.c:1242
 __x64_sys_ptrace+0x51/0x60 kernel/ptrace.c:1242
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 13229 Comm: syz-executor.0 Not tainted 5.8.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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