This one's persisted for a while, so time to file I guess ;)  Seen at least
through 2.6.17-1.2600.fc6.  Hit at the call to hwclock in rc.sysinit

BUG: warning at kernel/lockdep.c:1816/trace_hardirqs_on() (Not tainted)
 [<c04051ee>] show_trace_log_lvl+0x58/0x159
 [<c04057ea>] show_trace+0xd/0x10
 [<c0405903>] dump_stack+0x19/0x1b
 [<c043ac0a>] trace_hardirqs_on+0xa2/0x11e
 [<c0613f23>] _spin_unlock_irq+0x22/0x26
 [<c0541390>] rtc_get_rtc_time+0x32/0x176
 [<c0419ba0>] hpet_rtc_interrupt+0x92/0x14d
 [<c0450f78>] handle_IRQ_event+0x20/0x4d
 [<c0451039>] __do_IRQ+0x94/0xef
 [<c0406790>] do_IRQ+0xb9/0xcd
 [<c0404a49>] common_interrupt+0x25/0x2c
DWARF2 unwinder stuck at common_interrupt+0x25/0x2c
Leftover inexact backtrace:
 [<c04057ea>] show_trace+0xd/0x10
 [<c0405903>] dump_stack+0x19/0x1b
 [<c043ac0a>] trace_hardirqs_on+0xa2/0x11e
 [<c0613f23>] _spin_unlock_irq+0x22/0x26
 [<c0541390>] rtc_get_rtc_time+0x32/0x176
 [<c0419ba0>] hpet_rtc_interrupt+0x92/0x14d
 [<c0450f78>] handle_IRQ_event+0x20/0x4d
 [<c0451039>] __do_IRQ+0x94/0xef
 [<c0406790>] do_IRQ+0xb9/0xcd
 [<c0404a49>] common_interrupt+0x25/0x2c
 [<c0484c10>] sys_select+0x93/0x167
 [<c0403faf>] syscall_call+0x7/0xb