send_sig_info() has been broken since 2.5.60.

The function can be invoked from a the time interrupt (timer_interrpt ->
do_timer -> update_process_times -> -> update_one_process -> (
do_process_times, do_it_prof, do_it_virt ) -> -> send_sig ->
send_sig_info) but it uses spin_unlock_irq instead of the correct
spin_unlock_irqrestore. 

This enables interrupts, and later scheduler_tick() locks runqueue
(without disabling interrupts).  And if we are unlucky, a new interrupt
comes at this point.  And if this interrupt tries to do wake_up() (like
RTC interrupt does), we will deadlock on runqueue lock :-(

The bug was introduced by signal-fixes-2.5.59-A4, which split the
original send_sig_info into two functions, and in one branch it started
using these unsafe spinlock variants (while the "group" variant uses
irqsave/restore correctly).