Re: [PATCH] printk: robustify printk

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

On Fri, Aug 08, 2008 at 12:37:47PM -0700, Andrew Morton wrote:
> On Fri, 08 Aug 2008 21:21:08 +0200
> Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> 
> > On Fri, 2008-08-08 at 12:14 -0700, Andrew Morton wrote:
> > > On Fri, 08 Aug 2008 20:14:28 +0200
> > > Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > > 
> > > >  void wake_up_klogd(void)
> > > >  {
> > > > -	if (!oops_in_progress && waitqueue_active(&log_wait))
> > > > -		wake_up_interruptible(&log_wait);
> > > > +	unsigned long flags;
> > > > +	struct klogd_wakeup_state *kws;
> > > > +
> > > > +	if (!waitqueue_active(&log_wait))
> > > > +		return;
> > > > +
> > > > +	local_irq_save(flags);
> > > > +	kws = &__get_cpu_var(kws);
> > > > +	if (!kws->pending) {
> > > > +		kws->pending = 1;
> > > > +		call_rcu(&kws->head, __wake_up_klogd);
> > > > +	}
> > > > +	local_irq_restore(flags);
> > > >  }
> > > 
> > > Note that kernel/rcupreempt.c's flavour of call_rcu() takes
> > > RCU_DATA_ME().lock, so there are still code sites from which a printk
> > > can deadlock.  Only now, it is config-dependent.
> > > 
> > > From a quick look it appears that large amounts of kernel/rcupreempt.c
> > > are now a printk-free zone.
> > 
> > Drad, missed that bit, I did look at the calling end, but forgot the
> > call_rcu() end :-/
> > 
> > The initial printk_tick() based implementation didn't suffer this
> > problem, should we revert to that scheme?
> 
> Dunno.  Perhaps we could convert RCU_DATA_ME's spinlock_t into an
> rwlock and do read_lock() in call_rcu()?  Then we can should be able to
> call printk from inside that read_lock(), but not inside write_lock(),
> which, with suitable warning comments might be acceptable.
> 
> afacit everything in call_rcu()'s *rdp is cpu-local and is protected by
> local_irq_save().  rcu_ctrlblk.completed and rcu_flipped need some
> protection, but a) rdp->lock isn't sufficient anyway and b)
> read_lock protection would suffice.  Maybe other CPUs can alter *rdp
> while __rcu_advance_callbacks() is running.
> 
> Anyway, that's all handwaving.  My point is that making rcupreempt.c
> more robust and more concurrent might be an alternative fix, and might
> be beneficial in its own right.  Working out the details is what we
> have Pauls for ;)

How about if I instead add comments warning people not to put printk()
in the relevant RCU-implementation code?  That way I can be not only lazy,
but cowardly as well!  ;-)

							Thanx, Paul