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Date: Wed, 18 Jun 2014 09:43:27 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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Subject: Re: [RFC PATCH 00/11] printk: safe printing in NMI context
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On Wed, Jun 18, 2014 at 12:38:37PM -0400, Steven Rostedt wrote:
> On Wed, 18 Jun 2014 09:21:17 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> 
> > On Wed, Jun 18, 2014 at 05:58:40AM -1000, Linus Torvalds wrote:
> > > On Jun 18, 2014 4:36 AM, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > > wrote:
> > > >
> > > > I could easily add an option to RCU to allow people to tell it not to
> > > > use NMIs to dump the stack.
> > > 
> > > I don't think it should be an "option".
> > > 
> > > We should stop using nmi as if it was something "normal". It isn't. Code
> > > running in nmi context should be special, and should be very very aware
> > > that it is special. That goes way beyond "don't use printk". We seem to
> > > have gone way way too far in using nmi context.
> > > 
> > > So we should get *rid* of code in nmi context rather than then complain
> > > about printk being buggy.
> > 
> > OK, unconditional non-use of NMIs is even easier.  ;-)
> > 
> > Something like the following.
> 
> I have found the RCU stalls extremely useful in debugging lockups. In
> case this doesn't work as well, I'm willing to write up something that
> could send NMIs to all CPUs that would write into the ftrace ring
> buffer and when finished, the calling CPU can dump it out. No printk
> from NMI context at all.

Sounds like a good plan to me!

							Thanx, Paul