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BUG: KCSAN: data-race in __d_instantiate / path_lookupat

read-write to 0xffff88811b7b0cc0 of 4 bytes by task 29689 on cpu 0:
 fsnotify_update_flags include/linux/fsnotify_backend.h:-1 [inline]
 __d_instantiate+0x2a8/0x390 fs/dcache.c:1977
 d_make_persistent+0x78/0x110 fs/dcache.c:2793
 shmem_mknod+0x15f/0x170 mm/shmem.c:3892
 shmem_mkdir+0x33/0x70 mm/shmem.c:3933
 vfs_mkdir+0x2db/0x3d0 fs/namei.c:5130
 do_mkdirat+0x133/0x3b0 fs/namei.c:5164
 __do_sys_mkdir fs/namei.c:5191 [inline]
 __se_sys_mkdir fs/namei.c:5189 [inline]
 __x64_sys_mkdir+0x42/0x50 fs/namei.c:5189
 x64_sys_call+0x2f32/0x3000 arch/x86/include/generated/asm/syscalls_64.h:84
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xca/0x2b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811b7b0cc0 of 4 bytes by task 29691 on cpu 1:
 d_managed include/linux/dcache.h:403 [inline]
 step_into fs/namei.c:2078 [inline]
 walk_component fs/namei.c:2227 [inline]
 lookup_last fs/namei.c:2722 [inline]
 path_lookupat+0x1d8/0x500 fs/namei.c:2746
 filename_lookup+0x147/0x340 fs/namei.c:2775
 user_path_at+0x3e/0x130 fs/namei.c:3567
 do_mount fs/namespace.c:4032 [inline]
 __do_sys_mount fs/namespace.c:4224 [inline]
 __se_sys_mount+0x25b/0x2e0 fs/namespace.c:4201
 __x64_sys_mount+0x67/0x80 fs/namespace.c:4201
 x64_sys_call+0x2cca/0x3000 arch/x86/include/generated/asm/syscalls_64.h:166
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xca/0x2b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00000080 -> 0x08100080

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 29691 Comm: syz.9.5538 Tainted: G        W           syzkaller #0 PREEMPT(voluntary) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
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9p: Bad value for 'rfdno'