It looks like if the other thread opened the same file you might get a false positive.

    struct stat {
        dev_t     st_dev;     /* ID of device containing file */
        ino_t     st_ino;     /* inode number */
        mode_t    st_mode;    /* protection */
        nlink_t   st_nlink;   /* number of hard links */
        uid_t     st_uid;     /* user ID of owner */
        gid_t     st_gid;     /* group ID of owner */
        dev_t     st_rdev;    /* device ID (if special file) */
        off_t     st_size;    /* total size, in bytes */
        blksize_t st_blksize; /* blocksize for file system I/O */
        blkcnt_t  st_blocks;  /* number of 512B blocks allocated */
        time_t    st_atime;   /* time of last access */
        time_t    st_mtime;   /* time of last modification */
        time_t    st_ctime;   /* time of last status change */
    };

The only one of these likely to change is st_atime, but I don't think that's guaranteed to update. (I think different behaviours can be compiled into the linux kernel, including "only update atime if it equals ctime or mtime".)