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BUG: KCSAN: data-race in ptrace_check_attach / zap_other_threads

read-write to 0xffff888134b5c4a8 of 8 bytes by task 14110 on cpu 1:
 task_clear_jobctl_pending kernel/signal.c:344 [inline]
 zap_other_threads+0x90/0x1d0 kernel/signal.c:1356
 do_group_exit+0x140/0x170 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+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888134b5c4a8 of 8 bytes by task 14119 on cpu 0:
 ptrace_freeze_traced kernel/ptrace.c:178 [inline]
 ptrace_check_attach+0xdc/0x230 kernel/ptrace.c:247
 __do_sys_ptrace kernel/ptrace.c:1272 [inline]
 __se_sys_ptrace+0xdb/0x2e0 kernel/ptrace.c:1242
 __x64_sys_ptrace+0x51/0x60 kernel/ptrace.c:1242
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 14119 Comm: syz-executor.4 Not tainted 5.10.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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