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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Jun 18, 2014 at 04:53:14PM +0200, Jiri Kosina wrote:
> On Wed, 18 Jun 2014, Paul E. McKenney wrote:
> 
> > > > > - both RCU stall detector and 'echo l > sysrq-trigger' can (and we've 
> > > > >   seen it happening for real) cause a complete, undebuggable, silent hang 
> > > > >   of machine (deadlock in NMI context)
> > > > 
> > > > I could easily add an option to RCU to allow people to tell it not to
> > > > use NMIs to dump the stack.  Would that help?
> > > 
> > > Well, that would make unfortunately the information provided by RCU stall 
> > > detector rather useless ... workqueue-based stack dumping is very unlikely 
> > > to point its finger to the real offender, as it'd be coming way too late.
> > 
> > I would not use workqueues, but rather have the CPU detecting the
> > stall grovel through the other CPUs' stacks, which is what I do now for
> > architectures that don't support NMI-based stack dumps.  Would that be
> > a reasonable approach?
> 
> That would indeed solve lockups induced by RCU stall detector (and we 
> should convert sysrq stack dumping code to use the same mechanism 
> afterwards).
> 
> But then, the kernel is still polluted by quite a few instances of
> 
> 	WARN_ON(in_nmi())
> 
> 	BUG_IN(in_nmi())
> 
> 	if (in_nmi())
> 		printk(....)
> 
> which need to be fixed separately afterwards anyway.

True enough!

							Thanx, Paul