linux-kernel - Re: [RFC PATCH 00/11] printk: safe printing in NMI context

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Date:	Wed, 18 Jun 2014 09:43:27 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.cz>, Jan Kara <jack@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Dave Anderson <anderson@...hat.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Mladek <pmladek@...e.cz>, Kay Sievers <kay@...y.org>
Subject: Re: [RFC PATCH 00/11] printk: safe printing in NMI context

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@...ux.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@...ux.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

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