CPU package and APIC topology summary

The machine has two physical Xeon E5520 packages.

Observed ACPI CPU mapping:
CPU0  LAPIC 0   kernel CPU number 0
CPU1  LAPIC 16  kernel CPU number 8
CPU2  LAPIC 2   kernel CPU number 2
CPU3  LAPIC 18  kernel CPU number 10
CPU4  LAPIC 4   kernel CPU number 4
CPU5  LAPIC 20  kernel CPU number 12
CPU6  LAPIC 6   kernel CPU number 6
CPU7  LAPIC 22  kernel CPU number 14
CPU8  LAPIC 1   kernel CPU number 1
CPU9  LAPIC 17  kernel CPU number 9
CPUA  LAPIC 3   kernel CPU number 3
CPUB  LAPIC 19  kernel CPU number 11
CPUC  LAPIC 5   kernel CPU number 5
CPUD  LAPIC 21  kernel CPU number 13
CPUE  LAPIC 7   kernel CPU number 7
CPUF  LAPIC 23  kernel CPU number 15

Interpretation:
First physical package uses LAPIC IDs 0 to 7.
Second physical package uses LAPIC IDs 16 to 23.

The panic says:
pmLock: waited too long, held by 8

Kernel logical CPU 8 is the first logical CPU from the second physical package and corresponds to LAPIC ID 16.

This is why the failure appears correlated with activation of the second CPU package rather than simply with total CPU count.