x86, mtrr: Use stop machine context to rendezvous all the cpu's

x86, mtrr: Use stop machine context to rendezvous all the cpu's

Use the stop machine context rather than IPI's to rendezvous all the cpus for
MTRR initialization that happens during cpu bringup or for MTRR modifications
during runtime.

This avoids deadlock scenario (reported by Prarit) like:

cpu A holds a read_lock (tasklist_lock for example) with irqs enabled
cpu B waits for the same lock with irqs disabled using write_lock_irq
cpu C doing set_mtrr() (during AP bringup for example), which will try to
rendezvous all the cpus using IPI's

This will result in C and A come to the rendezvous point and waiting
for B. B is stuck forever waiting for the lock and thus not
reaching the rendezvous point.

Using stop cpu (run in the process context of per cpu based keventd) to do
this rendezvous, avoids this deadlock scenario.

Also make sure all the cpu's are in the rendezvous handler before we proceed
with the local_irq_save() on each cpu. This lock step disabling irqs on all
the cpus will avoid other deadlock scenarios (for example involving
with the blocking smp_call_function's etc).

   [ This problem is very old. Marking -stable only for 2.6.35 as the
     stop_one_cpu_nowait() API is present only in 2.6.35. Any older
     kernel interested in this fix need to do some more work in backporting
     this patch. ]

Reported-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
LKML-Reference: <1280515602.2682.10.camel@sbsiddha-MOBL3.sc.intel.com>
Acked-by: Prarit Bhargava <prarit@redhat.com>
Cc: stable@kernel.org	[2.6.35]
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>