Index: sys/powerpc/powerpc/swtch64.S
===================================================================
--- sys/powerpc/powerpc/swtch64.S	(revision 345243)
+++ sys/powerpc/powerpc/swtch64.S	(working copy)
@@ -122,13 +122,13 @@
 	mr	%r13,%r4		/* and the new thread ptr in curthread*/
 	mr	%r16,%r5		/* and the new lock */
 	mr	%r17,%r6		/* and the PCB */
+
+	//stdu	%r1,-48(%r1)		/* Reserve a stack frame to isolate stack. */ 
 	
-	stdu	%r1,-48(%r1)
-
 	lwz	%r7, PCB_FLAGS(%r17)
 	andi.	%r7, %r7, PCB_CDSCR
 	beq	.L0
-	/* Custom DSCR was set. Reseting it to enter kernel */
+	/* Custom DSCR was set. Resetting it to enter kernel */
 	li	%r7, 0x0
 	mtspr   SPR_DSCR, %r7
 
@@ -159,18 +159,30 @@
 cpu_switchin:
 #if defined(SMP) && defined(SCHED_ULE)
 	/* Wait for the new thread to become unblocked */
-	ld	%r6,TOC_REF(blocked_lock)(%r2)
+	addis	%r6,%r2,TOC_REF(blocked_lock)@ha
+	ld	%r6,TOC_REF(blocked_lock)@l(%r6)
 blocked_loop:
 	ld	%r7,TD_LOCK(%r13)
 	cmpd	%r6,%r7 
 	beq-	blocked_loop
 	isync
+	sync				/* Ensure PCB is synced. */ 
 #endif
 	
 	ld	%r17,TD_PCB(%r13)	/* Get new PCB */
-	ld	%r1,PCB_SP(%r17)	/* Load the stack pointer */
-	addi	%r1,%r1,-48		/* Remember about cpu_switch stack frame */
+	ld	%r7,PCB_SP(%r17)	/* Load the stack pointer */
+	cmpdi	%r7,0
+	bne+	0f
+	trap
 
+0:
+	//addi	%r7,%r7,-48		/* Remember about cpu_switch stack frame */
+	mr	%r1,%r7			/* Atomically switch stacks. */
+	//addi	%r1,%r1,-48		/* Remember about cpu_switch stack frame */
+	mflr	%r7
+	std	%r7,16(%r1)
+	stdu	%r1,-48(%r1)
+
 	/* Release old thread now that we have a stack pointer set up */
 	cmpdi	%r14,0
 	beq-	1f
@@ -212,7 +224,7 @@
 
 	/* thread to restore is in r3 */
 .L4:
-	addi	%r1,%r1,48
+	ld	%r1,0(%r1)		/* Remove the protection frame. */
 	mr	%r3,%r17		/* Recover PCB ptr */
 	ld	%r12,PCB_CONTEXT(%r3)	/* Load the non-volatile GP regs. */
 	ld	%r14,PCB_CONTEXT+2*8(%r3)