[PATCH] x86_64: Fix backtracing for interrupt stacks

Re-add backlink for old style unwinder to stack switching. Add proper
stack frame and CFI annotations to call_softirq

This prevents a oops when backtracing with fallback through the
interrupt stack top.

Suggested by Jan Beulich and Herbert Xu wanted it in 2.6.18.

Cc: [e-mail]
Signed-off-by: Andi Kleen <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/x86_64/kernel/entry.S

···
 	swapgs	
 1:	incl	%gs:pda_irqcount	# RED-PEN should check preempt count
 	cmoveq %gs:pda_irqstackptr,%rsp
+	push    %rbp			# backlink for old unwinder
 	/*
 	 * We entered an interrupt context - irqs are off:
 	 */
···
 END(machine_check)
 #endif
 
+/* Call softirq on interrupt stack. Interrupts are off. */
 ENTRY(call_softirq)
 	CFI_STARTPROC
-	movq %gs:pda_irqstackptr,%rax
-	movq %rsp,%rdx
-	CFI_DEF_CFA_REGISTER	rdx
+	push %rbp
+	CFI_ADJUST_CFA_OFFSET	8
+	CFI_REL_OFFSET rbp,0
+	mov  %rsp,%rbp
+	CFI_DEF_CFA_REGISTER rbp
 	incl %gs:pda_irqcount
-	cmove %rax,%rsp
-	pushq %rdx
-	/*todo CFI_DEF_CFA_EXPRESSION ...*/
+	cmove %gs:pda_irqstackptr,%rsp
+	push  %rbp			# backlink for old unwinder
 	call __do_softirq
-	popq %rsp
+	leaveq
 	CFI_DEF_CFA_REGISTER	rsp
+	CFI_ADJUST_CFA_OFFSET   -8
 	decl %gs:pda_irqcount
 	ret
 	CFI_ENDPROC