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BUG: KCSAN: data-race in __change_pid / next_tgid

write to 0xffff88812b069c28 of 8 bytes by task 8053 on cpu 1:
 __change_pid+0x10d/0x1e0 kernel/pid.c:341
 detach_pid+0x29/0x40 kernel/pid.c:352
 __unhash_process kernel/exit.c:77 [inline]
 __exit_signal kernel/exit.c:157 [inline]
 release_task+0x9ec/0xc60 kernel/exit.c:207
 wait_task_zombie kernel/exit.c:1080 [inline]
 wait_consider_task+0x1b3f/0x1c60 kernel/exit.c:1307
 do_wait_thread kernel/exit.c:1370 [inline]
 do_wait+0x1c9/0x4c0 kernel/exit.c:1441
 kernel_wait4+0x156/0x240 kernel/exit.c:1613
 call_usermodehelper_exec_sync kernel/umh.c:150 [inline]
 call_usermodehelper_exec_work+0x103/0x190 kernel/umh.c:187
 process_one_work+0x3d4/0x890 kernel/workqueue.c:2264
 worker_thread+0xa0/0x800 kernel/workqueue.c:2410
 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

read to 0xffff88812b069c28 of 8 bytes by task 9121 on cpu 0:
 task_tgid include/linux/sched/signal.h:606 [inline]
 has_group_leader_pid include/linux/sched/signal.h:642 [inline]
 next_tgid+0xf9/0x210 fs/proc/base.c:3301
 proc_pid_readdir+0x1f4/0x423 fs/proc/base.c:3339
 proc_root_readdir+0x8f/0xb0 fs/proc/root.c:263
 iterate_dir+0x312/0x380 fs/readdir.c:65
 __do_sys_getdents fs/readdir.c:285 [inline]
 __se_sys_getdents fs/readdir.c:266 [inline]
 __x64_sys_getdents+0x14b/0x280 fs/readdir.c:266
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 9121 Comm: ps Not tainted 5.5.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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