syzbot

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BUG: KCSAN: data-race in __follow_mount_rcu / do_new_mount

read to 0xffff888106803cc0 of 4 bytes by task 5162 on cpu 1:
 __follow_mount_rcu+0x15f/0x2b0 fs/namei.c:1709
 handle_mounts fs/namei.c:1735 [inline]
 step_into_slowpath+0x30c/0x4c0 fs/namei.c:2101
 step_into fs/namei.c:2149 [inline]
 walk_component fs/namei.c:2285 [inline]
 lookup_last fs/namei.c:2786 [inline]
 path_lookupat+0x330/0x500 fs/namei.c:2810
 filename_lookup+0x190/0x390 fs/namei.c:2839
 user_path_at+0x3e/0x100 fs/namei.c:3618
 __do_sys_pivot_root fs/namespace.c:4738 [inline]
 __se_sys_pivot_root fs/namespace.c:4726 [inline]
 __x64_sys_pivot_root+0x9f/0x110 fs/namespace.c:4726
 x64_sys_call+0x20a/0x3020 arch/x86/include/generated/asm/syscalls_64.h:156
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read-write to 0xffff888106803cc0 of 4 bytes by task 5161 on cpu 0:
 maybe_free_mountpoint fs/namespace.c:927 [inline]
 unpin_mountpoint fs/namespace.c:942 [inline]
 __unlock_mount fs/namespace.c:2792 [inline]
 unlock_mount fs/namespace.c:2800 [inline]
 do_new_mount_fc fs/namespace.c:3786 [inline]
 do_new_mount+0x6a8/0x8d0 fs/namespace.c:3839
 path_mount+0x4d0/0xbc0 fs/namespace.c:4159
 do_mount fs/namespace.c:4172 [inline]
 __do_sys_mount fs/namespace.c:4361 [inline]
 __se_sys_mount+0x28c/0x2e0 fs/namespace.c:4338
 __x64_sys_mount+0x67/0x80 fs/namespace.c:4338
 x64_sys_call+0x2d61/0x3020 arch/x86/include/generated/asm/syscalls_64.h:166
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 5161 Comm: syz.5.403 Tainted: G        W           syzkaller #0 PREEMPT(full) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
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