Deadlock in ia64_mca_cmc_int_caller

[Keith Owens <kaos@sgi.com>]

ia64_mca_cmc_int_caller() calls smp_call_function() which waits until
all cpus have taken the IPI before returning.  This interacts badly
with locks that are sometimes taken with interrupts disabled and
sometimes with interrupts enabled, smp_call_function can deadlock.

cpu 3                                                   cpu 0
Holds tasklist_lock with interrupts enabled,
it did read_lock() or write_lock().

                                                Does read_lock_irq() or
                                                write_lock_irq().  Spinning
                                                disabled waiting for tasklist_lock.

CMC interrupt occurs

ia64_mca_cmc_int_caller() calls smp_call_function()

smp_call_function() sends IPI to other cpus

                                                IPI on cpu 0 blocked, it is disabled
						waiting for tasklist_lock.

smp_call_function() waits until IPI reaches
all other cpus.

cpu 0 never responds, we never release the
tasklist lock, deadlock.

AFAICT it is never safe to call smp_call_function() from an interrupt
handler.

The unsafe nature of smp_call_function is not ia64 specific.  ix86 can
deadlock this way if any ix86 code calls smp_call_function from an
interrupt handler.