linux-kernel - Re: [PATCH RFC tip/core/rcu 04/41] rcu: Add diagnostic for misaligned rcu

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Date:	Thu, 2 Feb 2012 08:22:35 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH RFC tip/core/rcu 04/41] rcu: Add diagnostic for
 misaligned rcu_head structures

On Wed, Feb 01, 2012 at 05:00:26PM -0800, Josh Triplett wrote:
> On Wed, Feb 01, 2012 at 11:41:22AM -0800, Paul E. McKenney wrote:
> > The push for energy efficiency will require that RCU tag rcu_head
> > structures to indicate whether or not their invocation is time critical.
> > This tagging is best carried out in the bottom bits of the ->next
> > pointers in the rcu_head structures.  This tagging requires that the
> > rcu_head structures be properly aligned, so this commit adds the required
> > diagnostics.
> 
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -1707,6 +1707,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
> >  	unsigned long flags;
> >  	struct rcu_data *rdp;
> >  
> > +	WARN_ON_ONCE((unsigned long)head & 0x3); /* Misaligned rcu_head! */
> 
> I wonder if this should go out in one of the wrapper macros, so that the
> _ONCE means once per call site?

Certainly past experience introducing lockdep-RCU would argue that way.  ;-)

However, my testing thus far (famous last words!) has found no
misalignments.  So I expect that these will be caught as they are
inserted, so one at a time should be fine.

								Thanx, Paul

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