SMP - fixed IPI livelock

- two CPUs can issue IPI to each other now without any hazzard

- we must be able to handle synchronous scheduling IPIs from
  other CPUs when we are waiting for attention from another one.
  Otherwise we might livelock.

- necessary barriers to prevent reordering

kernel/arch/i386/arch_clock.c

@@ -17,8 +17,13 @@
 #ifdef USE_APIC
 #include "apic.h"
 #endif
+
 #include "spinlock.h"
 
+#ifdef CONFIG_SMP
+#include "kernel/smp.h"
+#endif
+
 #define CLOCK_ACK_BIT   0x80    /* PS/2 clock interrupt acknowledge bit */
 
 /* Clock parameters. */
@@ -235,6 +240,20 @@ PUBLIC void context_stop(struct proc * p)
 		bkl_succ[cpu] += !(!(succ == 0));
 
 		p->p_cycles = add64(p->p_cycles, sub64(tsc, *__tsc_ctr_switch));
+
+#ifdef CONFIG_SMP
+		/*
+		 * Since at the time we got a scheduling IPI we might have been
+		 * waiting for BKL already, we may miss it due to a similar IPI to
+		 * the cpu which is already waiting for us to handle its. This
+		 * results in a live-lock of these two cpus.
+		 *
+		 * Therefore we always check if there is one pending and if so,
+		 * we handle it straight away so the other cpu can continue and
+		 * we do not deadlock.
+		 */
+		smp_sched_handler();
+#endif
 	}
 #else
 	read_tsc_64(&tsc);