Problem with GDB when debugging IRQ handlers

[Catalin Marinas]

On Tue, Jun 28, 2011 at 03:20:45PM +0100, Catalin Marinas wrote:
> On Tue, Jun 28, 2011 at 04:06:11PM +0400, Dmitry Eremin-Solenikov wrote:
> > On 6/28/11, Russell King - ARM Linux <linux at arm.linux.org.uk> wrote:
> > I did some checks. It seems, the problem isn't related to unwinder. At least
> > it looks like kernel has all necessary unwinding subops. It looks like the
> > problem is really related to the lack of necessary .cfi information. At least
> > when i added .cfi_startproc/.cfi_endproc annotations to entry-armv.S code,
> > gdb stopped decoding backtrace with the "previous frame identical to this frame"
> > error. Unfortunately I don't have enough knowledge to add .cfi annotations to
> > irq handlers.
> 
> I think it may have stopped decoding because of some information it
> reads from the stack doesn't look sane. But I wonder whether we could
> get it looping again depending on the register values in the interrupted
> context.
> 
> > diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> > index e8d8856..d77f9d7 100644
> > --- a/arch/arm/kernel/entry-armv.S
> > +++ b/arch/arm/kernel/entry-armv.S
> > @@ -28,6 +28,7 @@
> >  #include "entry-header.S"
> >  #include <asm/entry-macro-multi.S>
> >  
> > +	.cfi_sections	.debug_frame
> >  /*
> >   * Interrupt handling.  Preserves r7, r8, r9
> >   */
> > @@ -113,6 +114,7 @@ ENDPROC(__und_invalid)
> >  
> >  	.macro	svc_entry, stack_hole=0
> >   UNWIND(.fnstart		)
> > +	.cfi_startproc
> >   UNWIND(.save {r0 - pc}		)
> >  	sub	sp, sp, #(S_FRAME_SIZE + \stack_hole - 4)
> 
> Could you add some directives like below in the svc_entry macro (after
> "sub sp...", not sure if it matters) and check whether gdb behaves
> better:
> 
> 	.cfi_def_cfa_offset S_PC
> 	.cfi_offset 14, -4

Actually since the return address is in S_PC (which maybe gdb assumes it
would be the saved LR), this is probably not be correct. After SVC
entry, we have he following structure on the stack:

	ORIG_r0
	CPSR
		<--- assuming this is the Call Frame Address (SP+S_PC+4)
	PC	<--- CFA - 4
	LR	<--- don't care
	SP	<--- CFA - 12
	...


So we tell gdb about this with something like below (untested):

	.cfi_def_cfa_offset S_PC + 4
	.cfi_offset 14, -4
	.cfi_offset 13, -12

-- 
Catalin