linux-kernel - Re: [PATCH 5/6] x86: Remove useless unwinder backlink from irq regs saving

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Message-ID: <20110704091742.GC22943@elte.hu>
Date:	Mon, 4 Jul 2011 11:17:42 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jan Beulich <JBeulich@...ell.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 5/6] x86: Remove useless unwinder backlink from irq regs
 saving


* Jan Beulich <JBeulich@...ell.com> wrote:

> >>> On 02.07.11 at 18:29, Frederic Weisbecker <fweisbec@...il.com> wrote:
> > The unwinder backlink in interrupt entry is very useless.
> > It's actually not part of the stack frame chain and thus is
> > never used.
> 
> I very much doubt this - see dump_trace()'s comment in its IRQ-stack
> related code portion (and the corresponding use of irq_stack_end[-1]).

> > +++ b/arch/x86/kernel/entry_64.S
> > @@ -327,7 +327,6 @@ ENDPROC(native_usergs_sysret64)
> >  	jne 2f
> >  	mov PER_CPU_VAR(irq_stack_ptr),%rsp
> >  	EMPTY_FRAME 0
> > -	pushq_cfi %rbp			/* backlink for unwinder */
> >  	/*
> >  	 * We entered an interrupt context - irqs are off:
> >  	 */

Frederic, please add it back with a much better comment in the .S 
showing where it's used and how. Perhaps even try to trigger the 
usage of this backlink and document the effect in the changelog.

Thanks,

	Ingo
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