Why some functions got WARNING L15, while others not!

Hi,
I have several functions called in T0 isr. The outline of call tree like this:

void T0_isr() interrupt 1 {
    getkey();
}

void getkey() {
    char c;
    c = filterGetKeyCode();
}

char filterGetKeyCode() {
    char c;
    c = getKeyCode();
    return c;
}

char getKeyCode() {
    char c;
    char cd;
    // c be assigned somehow
    cd = encodeFlex(c);
    c = decode(cd);
    return c;
}

char encodeFlex(char c) {
    return encode(c);
}

char encode(char);

char decode(char);

Now linker reports (simplified):
Warning 15: multiple call to segment
segment: getKeyCode
caller1: T0_isr
caller2: c_c51stratup

Warning 15: multiple call to segment
segment: encodeFlex
caller1: T0_isr
caller2: c_c51startup

My question is: why getKeyCode and encodeFlex incur warnings, while others, like decode, passed by linker.
When I disoverlay getKeyCode, and declare encodeFlex with reentrant, the warnings all disappear. But I still worry about other functions called in isr. Is it safe now?

Thanks in advance.

d.curie