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BUG: KCSAN: data-race in free_pid / zap_pid_ns_processes

write to 0xffff88811c551798 of 4 bytes by task 9289 on cpu 0:
 free_pid+0x79/0x190 kernel/pid.c:136
 __change_pid kernel/pid.c:353 [inline]
 detach_pid+0x147/0x160 kernel/pid.c:358
 __unhash_process kernel/exit.c:77 [inline]
 __exit_signal kernel/exit.c:148 [inline]
 release_task+0x6f0/0xbe0 kernel/exit.c:199
 exit_notify kernel/exit.c:699 [inline]
 do_exit+0xee9/0x1560 kernel/exit.c:845
 do_group_exit+0xce/0x1a0 kernel/exit.c:922
 get_signal+0xf83/0x15d0 kernel/signal.c:2781
 arch_do_signal_or_restart+0x2a/0x220 arch/x86/kernel/signal.c:789
 handle_signal_work kernel/entry/common.c:147 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:171 [inline]
 exit_to_user_mode_prepare+0x104/0x170 kernel/entry/common.c:208
 __syscall_exit_to_user_mode_work kernel/entry/common.c:290 [inline]
 syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:301
 do_syscall_64+0x40/0x50 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88811c551798 of 4 bytes by task 1764 on cpu 1:
 zap_pid_ns_processes+0x256/0x370 kernel/pid_namespace.c:244
 find_child_reaper kernel/exit.c:542 [inline]
 forget_original_parent kernel/exit.c:631 [inline]
 exit_notify kernel/exit.c:668 [inline]
 do_exit+0x13f0/0x1560 kernel/exit.c:845
 do_group_exit+0xa4/0x1a0 kernel/exit.c:922
 __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

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 1764 Comm: syz-executor.2 Not tainted 5.12.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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