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BUG: KCSAN: data-race in detach_pid / proc_pid_status

write to 0xffff88810d46c5a0 of 8 bytes by task 4894 on cpu 0:
 __change_pid kernel/pid.c:347 [inline]
 detach_pid+0xd7/0x160 kernel/pid.c:358
 __unhash_process kernel/exit.c:74 [inline]
 __exit_signal kernel/exit.c:147 [inline]
 release_task+0x610/0xd30 kernel/exit.c:198
 wait_task_zombie kernel/exit.c:1088 [inline]
 wait_consider_task+0xf89/0x1820 kernel/exit.c:1315
 do_wait_thread kernel/exit.c:1378 [inline]
 do_wait+0x1b1/0x4a0 kernel/exit.c:1449
 kernel_wait4+0x13c/0x1a0 kernel/exit.c:1611
 __do_sys_wait4 kernel/exit.c:1639 [inline]
 __se_sys_wait4 kernel/exit.c:1635 [inline]
 __x64_sys_wait4+0x7a/0x100 kernel/exit.c:1635
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88810d46c5a0 of 8 bytes by task 4869 on cpu 1:
 pid_alive include/linux/sched.h:1397 [inline]
 task_state fs/proc/array.c:163 [inline]
 proc_pid_status+0xb1/0x1350 fs/proc/array.c:414
 proc_single_show+0x84/0x100 fs/proc/base.c:775
 seq_read+0x2de/0x890 fs/seq_file.c:208
 vfs_read+0x154/0x5c0 fs/read_write.c:494
 ksys_read+0xce/0x180 fs/read_write.c:634
 __do_sys_read fs/read_write.c:644 [inline]
 __se_sys_read fs/read_write.c:642 [inline]
 __x64_sys_read+0x3e/0x50 fs/read_write.c:642
 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: 1 PID: 4869 Comm: systemd-journal Not tainted 5.9.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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