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BUG: KCSAN: data-race in ptrace_check_attach / zap_other_threads

write to 0xffff88813d541558 of 8 bytes by task 12313 on cpu 0:
 task_clear_jobctl_pending kernel/signal.c:344 [inline]
 zap_other_threads+0x98/0x1e0 kernel/signal.c:1356
 do_group_exit+0x145/0x1a0 kernel/exit.c:917
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:933
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:931
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:931
 do_syscall_64+0x34/0x50 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88813d541558 of 8 bytes by task 12318 on cpu 1:
 ptrace_freeze_traced kernel/ptrace.c:178 [inline]
 ptrace_check_attach+0x144/0x250 kernel/ptrace.c:247
 __do_sys_ptrace kernel/ptrace.c:1266 [inline]
 __se_sys_ptrace+0xd8/0x1b0 kernel/ptrace.c:1236
 __x64_sys_ptrace+0x51/0x60 kernel/ptrace.c:1236
 do_syscall_64+0x34/0x50 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 12318 Comm: syz-executor.4 Not tainted 5.12.0-rc8-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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