public inbox for linux-arch@vger.kernel.org
 help / color / mirror / Atom feed
From: Lai Jiangshan <laijs@cn.fujitsu.com>
To: Andrew Morton <akpm@linux-foundation.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	linux-arch@vger.kernel.org
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: [RFC PATCH]rcu,alpha: avoid smp_read_barrier_depends in rcu_dereference when pointer==NULL
Date: Sat, 21 Jun 2008 17:54:01 +0800	[thread overview]
Message-ID: <485CCFB9.9050208@cn.fujitsu.com> (raw)

Hi, Paul and Alpha hackers

memory barrier is expensive, I think it's worth that adding an if-statement
to avoid it when pointer == NULL.

This patch change nothing in UP or in other architectures for compiler's
optimization.

			Thanks, Lai Jiangshan


Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index d42dbec..8979f29 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -143,7 +143,8 @@ struct rcu_head {
 
 #define rcu_dereference(p)     ({ \
 				typeof(p) _________p1 = ACCESS_ONCE(p); \
-				smp_read_barrier_depends(); \
+				if (_________p1) \
+					smp_read_barrier_depends(); \
 				(_________p1); \
 				})
 

             reply	other threads:[~2008-06-21  9:55 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-06-21  9:54 Lai Jiangshan [this message]
2008-06-21 12:52 ` [RFC PATCH]rcu,alpha: avoid smp_read_barrier_depends in rcu_dereference when pointer==NULL Peter Zijlstra
2008-06-21 19:26   ` Paul E. McKenney

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=485CCFB9.9050208@cn.fujitsu.com \
    --to=laijs@cn.fujitsu.com \
    --cc=akpm@linux-foundation.org \
    --cc=linux-arch@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=paulmck@linux.vnet.ibm.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox