syzbot

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BUG: KCSAN: data-race in __d_lookup_rcu / __d_rehash

read to 0xffff88811f17e6d0 of 8 bytes by task 3101 on cpu 1:
 hlist_bl_unhashed include/linux/list_bl.h:54 [inline]
 d_unhashed include/linux/dcache.h:336 [inline]
 __d_lookup_rcu+0x120/0x290 fs/dcache.c:2399
 lookup_fast+0x8e/0x290 fs/namei.c:1629
 walk_component fs/namei.c:1998 [inline]
 link_path_walk+0x3f4/0x7e0 fs/namei.c:2329
 path_lookupat+0x72/0x2a0 fs/namei.c:2482
 filename_lookup+0x126/0x300 fs/namei.c:2512
 user_path_at_empty+0x42/0x110 fs/namei.c:2911
 user_path_at include/linux/namei.h:57 [inline]
 ksys_umount fs/namespace.c:1916 [inline]
 __do_sys_umount fs/namespace.c:1924 [inline]
 __se_sys_umount fs/namespace.c:1922 [inline]
 __x64_sys_umount+0x88/0xe0 fs/namespace.c:1922
 do_syscall_x64 arch/x86/entry/common.c:51 [inline]
 do_syscall_64+0x44/0x110 arch/x86/entry/common.c:82
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

write to 0xffff88811f17e6d0 of 8 bytes by task 3108 on cpu 0:
 hlist_bl_add_head_rcu include/linux/rculist_bl.h:81 [inline]
 __d_rehash+0xc4/0x220 fs/dcache.c:2578
 __d_add+0x36d/0x4a0 fs/dcache.c:2793
 d_splice_alias+0xd6/0x270 fs/dcache.c:3181
 ext4_lookup+0x199/0x390 fs/ext4/namei.c:1883
 lookup_one_qstr_excl+0xc0/0x180 fs/namei.c:1609
 filename_create+0x14d/0x2b0 fs/namei.c:3876
 do_mkdirat+0x63/0x2a0 fs/namei.c:4121
 __do_sys_mkdirat fs/namei.c:4144 [inline]
 __se_sys_mkdirat fs/namei.c:4142 [inline]
 __x64_sys_mkdirat+0x50/0x60 fs/namei.c:4142
 do_syscall_x64 arch/x86/entry/common.c:51 [inline]
 do_syscall_64+0x44/0x110 arch/x86/entry/common.c:82
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 3108 Comm: syz-executor.4 Not tainted 6.7.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/10/2023
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