linux-kernel - Re: [PATCH tip/core/rcu 1/3] ftrace: replace read_barrier_depends() with rcu_dereference

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Message-ID: <1267838945.10871.1786.camel@gandalf.stny.rr.com>
Date:	Fri, 05 Mar 2010 20:29:05 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH tip/core/rcu 1/3] ftrace: replace
 read_barrier_depends() with rcu_dereference_raw()

On Fri, 2010-03-05 at 15:03 -0800, Paul E. McKenney wrote:
> Replace the calls to read_barrier_depends() in ftrace_list_func() with
> rcu_dereference_raw() to improve readability.  The reason that we use
> rcu_dereference_raw() here is that removed entries are never freed,
> instead they are simply leaked.  This is one of a very few cases where
> use of rcu_dereference_raw() is the long-term right answer.  And I don't
> yet know of any others.  ;-)
> 
> Cc: Steven Rostedt <rostedt@...dmis.org>

Acked-by: Steven Rostedt <rostedt@...dmis.org>

Thanks Paul!

> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---

>  
> @@ -154,8 +159,7 @@ static int __register_ftrace_function(struct ftrace_ops *ops)
>  	 * the ops->next pointer is valid before another CPU sees
>  	 * the ops pointer included into the ftrace_list.
>  	 */
> -	smp_wmb();
> -	ftrace_list = ops;
> +	rcu_assign_pointer(ftrace_list, ops);

[ Off topic ]

I looked at rcu_assign_pointer() and it is:

#define rcu_assign_pointer(p, v) \
	({ \
		if (!__builtin_constant_p(v) || \
		    ((v) != NULL)) \
			smp_wmb(); \
		(p) = (v); \
	})

My question is, why that crazy if? The only time that will fail is if we
are assigning the constant NULL to p. What makes NULL so important here?
Can't there be a case when assigning NULL to p will require that wmb()?

-- Steve


>  
>  	if (ftrace_enabled) {
>  		ftrace_func_t func;


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