Not sure whether this is a problem or not, but when compiling some custom module
using the stock kernel-source-2.4.20-2.10 from Rawhide, I get

In file included from /usr/src/linux-2.4/include/linux/mm.h:22,
                 from /usr/src/linux-2.4/include/linux/slab.h:14,
                 from /usr/src/linux-2.4/include/asm/pci.h:32,
                 from /usr/src/linux-2.4/include/linux/pci.h:657,
                 from pcibus.c:5:
/usr/src/linux-2.4/include/linux/sched.h:548:1: warning: "cpu_online" redefined
In file included from /usr/src/linux-2.4/include/linux/sched.h:25,
                 from /usr/src/linux-2.4/include/linux/mm.h:22,
                 from /usr/src/linux-2.4/include/linux/slab.h:14,
                 from /usr/src/linux-2.4/include/asm/pci.h:32,
                 from /usr/src/linux-2.4/include/linux/pci.h:657,
                 from pcibus.c:5:
/usr/src/linux-2.4/include/linux/smp.h:87:1: warning: this is the location of
the previous definition

Indeed,  grep shows:

/usr/src/linux-2.4/include/linux/sched.h:548:#define cpu_online(cpu) ((cpu) <
smp_num_cpus)
/usr/src/linux-2.4/include/linux/smp.h:87:#define cpu_online(cpu)              
    ({ BUG_ON((cpu) != 0); 1; })