linux-kernel - Re: [PATCH -rc] [BUGFIX] x86: fix kernel_trap

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Date:	Tue, 12 May 2009 00:40:01 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Masami Hiramatsu <mhiramat@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	systemtap@...rces.redhat.com,
	Harvey Harrison <harvey.harrison@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jan Blunck <jblunck@...e.de>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH -rc] [BUGFIX] x86: fix kernel_trap_sp()

* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Mon, 11 May 2009, Masami Hiramatsu wrote:
> >
> > Use &regs->sp instead of regs for getting the top of stack in kernel mode.
> > (on x86-64, regs->sp always points the top of stack)
> 
> Ack. 
> 
> That said, we have only _one_ use of this "kernel_trap_sp()" in 
> the whole kernel, and that use is actually fairly odd too, in that 
> it does it _before_ checking that it's in kernel mode.
> 
> Admittedly it will then only _use_ the value after it has checked 
> that things are in kernel mode, but it all boils down to "ok, 
> that's pretty odd".
> 
> So how about fixing that, and also fixing the naming of the 
> function. Call it "kernel_stack_pointer()" to match its more 
> widely used sibling function "user_stack_pointer()".
> 
> IOW, something like this?

yeah, this is cleaner and probably a tad faster. I've applied it to 
x86/urgent with your acked-by - or would you like to apply this 
straight away?

The use of the backtracing feature on the oprofile side is not very 
common (and the bug is ancient) so i wanted to wait a few days with 
this.

One small detail:

> +	return (unsigned long)(&regs->sp);

the original commit had:

> +	return (unsigned long)&regs->sp;

and this latter is correct as well due to operator priorities. No 
strong preference from my side so i took your version as-is. I kept 
Masami as the author and a comment for your changes.

( Not sure how to express this in a changelog properly - i didnt 
  want two commits but it was authored by two people in essence. So 
  i went for the version that will show up in the commit 
  notification email in a few minutes - also attached below. )

	Ingo
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