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BUG: KCSAN: data-race in __dentry_kill / d_alloc_parallel

read to 0xffff888089d23840 of 4 bytes by task 7978 on cpu 0:
 dentry_unlist fs/dcache.c:544 [inline]
 __dentry_kill+0x1be/0x3c0 fs/dcache.c:575
 dentry_kill fs/dcache.c:698 [inline]
 dput+0x3b5/0x750 fs/dcache.c:859
 __lookup_slow+0x271/0x2a0 fs/namei.c:1668
 lookup_slow+0x4f/0x70 fs/namei.c:1682
 walk_component+0x490/0xd90 fs/namei.c:1802
 link_path_walk.part.0+0x5d3/0xa90 fs/namei.c:2128
 link_path_walk fs/namei.c:2059 [inline]
 path_mountpoint+0x94/0xdb0 fs/namei.c:2701
 filename_mountpoint+0x134/0x270 fs/namei.c:2724
 user_path_mountpoint_at+0x43/0x60 fs/namei.c:2754
 ksys_umount+0xdb/0xab0 fs/namespace.c:1683
 __do_sys_umount fs/namespace.c:1709 [inline]
 __se_sys_umount fs/namespace.c:1707 [inline]
 __x64_sys_umount+0x3a/0x50 fs/namespace.c:1707
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff888089d23840 of 4 bytes by task 10308 on cpu 1:
 d_alloc_parallel+0x533/0xe00 fs/dcache.c:2612
 __lookup_slow+0xb4/0x2a0 fs/namei.c:1648
 lookup_slow+0x4f/0x70 fs/namei.c:1682
 walk_component+0x490/0xd90 fs/namei.c:1802
 link_path_walk.part.0+0x5d3/0xa90 fs/namei.c:2128
 link_path_walk fs/namei.c:2059 [inline]
 path_openat+0x14f/0x3580 fs/namei.c:3536
 do_filp_open+0x11e/0x1b0 fs/namei.c:3567
 do_sys_open+0x3b3/0x4f0 fs/open.c:1097
 __do_sys_open fs/open.c:1115 [inline]
 __se_sys_open fs/open.c:1110 [inline]
 __x64_sys_open+0x55/0x70 fs/open.c:1110
 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: 1 PID: 10308 Comm: syz-executor.4 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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