From linux-kernel-owner+greg=40kroah.com-S1751243AbWA3FTq@vger.kernel.org Sun Jan 29 21:20:33 2006
Message-ID: <43DDA1E7.5010109@cosmosbay.com>
Date: Mon, 30 Jan 2006 06:19:35 +0100
From: Eric Dumazet <dada1@cosmosbay.com>
To: Greg KH <greg@kroah.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>, Herbert Poetzl <herbert@13thfloor.at>, "Serge E. Hallyn" <serue@us.ibm.com>, Alan Cox <alan@lxorguk.ukuu.org.uk>, Dave Hansen <haveblue@us.ibm.com>, Arjan van de Ven <arjan@infradead.org>, Suleiman Souhlal <ssouhlal@FreeBSD.org>, Hubertus Franke <frankeh@watson.ibm.com>, Cedric Le Goater <clg@fr.ibm.com>, Kyle Moffett <mrmacman_g4@mac.com>
Subject: kref: avoid an atomic operation in kref_put()

Avoid an atomic operation in kref_put() when the last reference is
dropped. On most platforms, atomic_read() is a plan read of the counter
and involves no atomic at all.


Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
 lib/kref.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- gregkh-2.6.orig/lib/kref.c
+++ gregkh-2.6/lib/kref.c
@@ -52,7 +52,12 @@ int kref_put(struct kref *kref, void (*r
 	WARN_ON(release == NULL);
 	WARN_ON(release == (void (*)(struct kref *))kfree);
 
-	if (atomic_dec_and_test(&kref->refcount)) {
+	/*
+	 * if current count is one, we are the last user and can release object
+	 * right now, avoiding an atomic operation on 'refcount'
+	 */
+	if ((atomic_read(&kref->refcount) == 1) ||
+	    (atomic_dec_and_test(&kref->refcount))) {
 		release(kref);
 		return 1;
 	}