linux-kernel - Re: [RFC/INCOMPLETE 01/13] context_tracking: Add context_tracking_assert

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Date:	Wed, 17 Jun 2015 08:27:56 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Borislav Petkov <bp@...en8.de>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: [RFC/INCOMPLETE 01/13] context_tracking: Add
 context_tracking_assert_state

On Wed, Jun 17, 2015 at 11:41:14AM +0200, Ingo Molnar wrote:
> 
> * Andy Lutomirski <luto@...nel.org> wrote:
> 
> > This will let us sprinkle sanity checks around the kernel without
> > making too much of a mess.
> > 
> > Signed-off-by: Andy Lutomirski <luto@...nel.org>
> > ---
> >  include/linux/context_tracking.h | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
> > index 2821838256b4..0fbea4b152e1 100644
> > --- a/include/linux/context_tracking.h
> > +++ b/include/linux/context_tracking.h
> > @@ -57,6 +57,13 @@ static inline void context_tracking_task_switch(struct task_struct *prev,
> >  	if (context_tracking_is_enabled())
> >  		__context_tracking_task_switch(prev, next);
> >  }
> > +
> > +static inline void context_tracking_assert_state(enum ctx_state state)
> > +{
> > +	rcu_lockdep_assert(!context_tracking_is_enabled() ||
> > +			   this_cpu_read(context_tracking.state) == state,
> > +			   "context tracking state was wrong");
> > +}
> 
> Please don't introduce assert() style debug check interfaces!
> 
> (And RCU should be fixed too I suspect.)

The thought is to rename rcu_lockdep_assert() to RCU_LOCKDEP_WARN()
by analogy to WARN()?  Easy to do if so!  Or am I missing the point?

						Thanx, Paul

> They are absolutely horrible on the brain when mixed with WARN_ON() interfaces, 
> which are the dominant runtime check interface in the kernel.
> 
> Instead make it something like:
> 
>   #define ct_state() (this_cpu_read(context_tracking.state))
> 
>   #define CT_WARN_ON(cond) \
> 	WARN_ON(context_tracking_is_enabled() && (cond))
> 
> and then the debug checks can be written as:
> 
> 	CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
> 
> This is IMHO _far_ more readable than:
> 
> 	context_tracking_assert_state(CONTEXT_KERNEL);
> 
> ok?
> 
> (Assuming people will accept 'ct/CT' as an abbreviation for context tracking.)
> 
> Thanks,
> 
> 	Ingo
> 

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