linux-kernel - Re: Dealing with the NMI mess

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Message-ID: <20150724205119.GM19282@twins.programming.kicks-ass.net>
Date:	Fri, 24 Jul 2015 22:51:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Willy Tarreau <w@....eu>, Steven Rostedt <rostedt@...dmis.org>,
	Andy Lutomirski <luto@...capital.net>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: Dealing with the NMI mess

On Fri, Jul 24, 2015 at 01:22:11PM -0700, Linus Torvalds wrote:
> On Fri, Jul 24, 2015 at 12:55 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > I worry that we'll end up running the do_debug() handlers from effective
> > NMI context.
> >
> > The NMI might have preempted locks which these handlers require etc..
> 
> If #DB takes any locks like that, then #DB is broken.
> 
> Pretty much by definition, a data breakpoint can happen on pretty much
> absolutely any code. This is in no way NMI-specific as far as I can
> tell.
> 
> Do we really take locks in the #DB handler?

do_debug()
  send_sigtrap()
    force_sig_info()
      spin_lock_irqsave()

Now, I don't pretend to understand the condition before send_sigtrap(),
so it _might_ be ok, but it sure as heck could do with a comment.
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