Date: Thu, 2 Feb 2012 08:22:35 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Josh Triplett <josh@joshtriplett.org>
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@polymtl.ca, niv@us.ibm.com, tglx@linutronix.de,
        peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu,
        dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com,
        fweisbec@gmail.com, patches@linaro.org,
        "Paul E. McKenney" <paul.mckenney@linaro.org>
Subject: Re: [PATCH RFC tip/core/rcu 04/41] rcu: Add diagnostic for
 misaligned rcu_head structures
Message-ID: <20120202162235.GI2518@linux.vnet.ibm.com>
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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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