I am trying to port the Linux 2.6.11-1 32-bit kernel to a HP server (Dual Intel XEON L5520 physical CPU). The APIC ID of each processor is 0-7 on the first physical CPU and 16-23 on the 2nd physical CPU. The boot processor's APIC ID is 16.

If I bring up more than 8 processors in smpboot.c, the kernel is deadlocked at wait_for_completion() in kernel/rcupdate.c while registering network protocol in net/ipv4/af_inet.c. If I bring up less than 9 processors (no matter these processors are from 1st or 2nd or both physical CPUs), the system work fine. Could you shed some light on this issue. Thank you very much.

The 2.6.11 kernel is four years older than your processors.
In other words: just use 2.6.31.