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BUG: KCSAN: data-race in __d_drop / __d_lookup

write to 0xffff888102be63d0 of 8 bytes by task 1785 on cpu 0:
 __hlist_bl_del include/linux/list_bl.h:128 [inline]
 ___d_drop fs/dcache.c:475 [inline]
 __d_drop+0x10c/0x1e0 fs/dcache.c:482
 d_invalidate+0x4a/0x1d0 fs/dcache.c:1705
 proc_invalidate_siblings_dcache+0x203/0x2f0 fs/proc/inode.c:150
 proc_flush_pid+0x16/0x20 fs/proc/base.c:3325
 release_task+0xad7/0xbe0 kernel/exit.c:223
 wait_task_zombie kernel/exit.c:1108 [inline]
 wait_consider_task+0x10ce/0x19e0 kernel/exit.c:1335
 do_wait_thread kernel/exit.c:1398 [inline]
 do_wait+0x1d1/0x6b0 kernel/exit.c:1515
 kernel_wait4+0x13c/0x1a0 kernel/exit.c:1678
 __do_sys_wait4 kernel/exit.c:1706 [inline]
 __se_sys_wait4 kernel/exit.c:1702 [inline]
 __x64_sys_wait4+0x7a/0x100 kernel/exit.c:1702
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff888102be63d0 of 8 bytes by task 1801 on cpu 1:
 hlist_bl_unhashed include/linux/list_bl.h:54 [inline]
 d_unhashed include/linux/dcache.h:342 [inline]
 __d_lookup+0x15d/0x400 fs/dcache.c:2431
 lookup_fast+0x67/0x350 fs/namei.c:1587
 walk_component+0x5c/0x350 fs/namei.c:1938
 link_path_walk+0x4ba/0x780 fs/namei.c:2265
 path_lookupat+0x7b/0x570 fs/namei.c:2420
 filename_lookup+0xff/0x390 fs/namei.c:2454
 user_path_at_empty+0x3b/0x50 fs/namei.c:2734
 user_path_at include/linux/namei.h:60 [inline]
 ksys_umount fs/namespace.c:1776 [inline]
 __do_sys_umount fs/namespace.c:1784 [inline]
 __se_sys_umount fs/namespace.c:1782 [inline]
 __x64_sys_umount+0x75/0xd0 fs/namespace.c:1782
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 1801 Comm: syz-executor.4 Not tainted 5.13.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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