linux-kernel - Re: [PATCH] kernel: events: core: Replace rcu_assign_pointer() with RCU_INIT

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Date:	Tue, 9 Sep 2014 11:42:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andreea-Cristina Bernat <bernat.ada@...il.com>
Cc:	paulus@...ba.org, mingo@...hat.com, acme@...nel.org,
	linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH] kernel: events: core: Replace rcu_assign_pointer() with
 RCU_INIT_POINTER()

On Fri, Aug 22, 2014 at 04:26:05PM +0300, Andreea-Cristina Bernat wrote:
> The use of "rcu_assign_pointer()" is NULLing out the pointer.
> According to RCU_INIT_POINTER()'s block comment:
> "1.   This use of RCU_INIT_POINTER() is NULLing out the pointer"
> it is better to use it instead of rcu_assign_pointer() because it has a
> smaller overhead.
> 
> The following Coccinelle semantic patch was used:
> @@
> @@
> 
> - rcu_assign_pointer
> + RCU_INIT_POINTER
>   (..., NULL)
> 
> Signed-off-by: Andreea-Cristina Bernat <bernat.ada@...il.com>
> ---
>  kernel/events/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 0d735be..4b84dd5 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -5744,7 +5744,7 @@ static void swevent_hlist_release(struct swevent_htable *swhash)
>  	if (!hlist)
>  		return;
>  
> -	rcu_assign_pointer(swhash->swevent_hlist, NULL);
> +	RCU_INIT_POINTER(swhash->swevent_hlist, NULL);
>  	kfree_rcu(hlist, rcu_head);
>  }

Paul, why not do something like the below and do away with all this
nonsense?

---
 include/linux/rcupdate.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index d231aa17b1d7..38c3e629ae70 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -623,7 +623,13 @@ static inline void rcu_preempt_sleep_check(void)
  * please be careful when making changes to rcu_assign_pointer() and the
  * other macros that it invokes.
  */
-#define rcu_assign_pointer(p, v) smp_store_release(&p, RCU_INITIALIZER(v))
+#define rcu_assign_pointer(p, v)					\
+do {									\
+	if (__builtin_constant_p(v) && v == NULL)			\
+		p = RCU_INITIALIZER(v);					\
+	else								\
+		smp_store_release(&p, RCU_INITIALIZER(v));		\
+} while (0)
 
 /**
  * rcu_access_pointer() - fetch RCU pointer with no dereferencing

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