To eliminate atomic lock contention along hot directory trees (/usr, /lib, /etc), the Linux kernel splits pathname resolution into two distinct execution modes: RCU-Walk (lockless, non-blocking traversal) and Ref-Walk (spinlock-backed reference counting).

1. struct nameidata and the Dual-Mode Lookup State Machine

Every VFS path lookup allocates a stack-local struct nameidata instance that tracks the current struct path (mnt + dentry), traversal flags (LOOKUP_RCU), and saved sequence counters (seq and m_seq).

// Simplified from fs/namei.c
struct nameidata {
    struct path     path;
    struct qstr     last;
    struct path     root;
    struct inode    *inode;
    unsigned int    flags;
    unsigned        seq, next_seq, m_seq, r_seq;
    int             last_type;
    unsigned        depth;
};