From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756024AbaIIJmq (ORCPT ); Tue, 9 Sep 2014 05:42:46 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:57218 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753881AbaIIJmo (ORCPT ); Tue, 9 Sep 2014 05:42:44 -0400 Date: Tue, 9 Sep 2014 11:42:35 +0200 From: Peter Zijlstra To: Andreea-Cristina Bernat Cc: paulus@samba.org, mingo@redhat.com, acme@kernel.org, linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com Subject: Re: [PATCH] kernel: events: core: Replace rcu_assign_pointer() with RCU_INIT_POINTER() Message-ID: <20140909094235.GD19379@twins.programming.kicks-ass.net> References: <20140822132605.GA20130@ada> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="1ExXuwkHt/ur4MCY" Content-Disposition: inline In-Reply-To: <20140822132605.GA20130@ada> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --1ExXuwkHt/ur4MCY Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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. >=20 > The following Coccinelle semantic patch was used: > @@ > @@ >=20 > - rcu_assign_pointer > + RCU_INIT_POINTER > (..., NULL) >=20 > Signed-off-by: Andreea-Cristina Bernat > --- > kernel/events/core.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) >=20 > 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_ht= able *swhash) > if (!hlist) > return; > =20 > - 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 =3D=3D NULL) \ + p =3D RCU_INITIALIZER(v); \ + else \ + smp_store_release(&p, RCU_INITIALIZER(v)); \ +} while (0) =20 /** * rcu_access_pointer() - fetch RCU pointer with no dereferencing --1ExXuwkHt/ur4MCY Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) iQIcBAEBAgAGBQJUDsuLAAoJEHZH4aRLwOS61A0P/RyDPsGVi/i0vSTuYouNhhA8 idGm7xanORRa5J7mmqu8kwbmo60Fc+asewHKEE5yJt4oBMwvAipKcqSaimDA43Bu 2ldsRPtOPNwsBJ/pp9rBFPUbgpWHr+8C0rBV5qWWVtldzrHmzuaE+ItZ9QE5MGqI GlvC2aXARc6WcnFZqXwki110nZC3OC+ggBhcFhEqW1cvNqlVg0qx+KlYJqJI1pDg biWrjRn+f1s3498uct/LzM9fuQorS5lr3Qc4+M1Tu4LbckGlkNQVLh3GuVdzfh5N mEl8CBkWtQrhdCT6A+2KuPB2nyRxaRTDkwMBJl7f4h2IjfucIZR5yhsUfGz40jce lB6j2FKPo/9BPx4D7IGlDmws8Q3QGQphWdP5JoZZOGs38PzqWpk19abQnBHtN32f YgPqE1vHnN96DxgjuPdIL2qWGSglMAP9CDR9ahE3bbWqd1kepGT4ohr4diJHT0o5 5EP7wVOZjh6AU3Q71qqfKUDFeAQtWxCQkfAG/uxDVEDrh4HwGhWYAiikIADlyBUK iurfNfbqLxYJA2ZrKaYwiBrE6XpRr88EyukcMFcGt27dL8GpvPfv0w4kdGVla++B 2KFR08beypx3npLNqrfgHOh6MP06vDEtOrmqoowRT5E5sp7RsKU3USwc+c4UEIJA 5OOm6+nK+MIJ4l5fhaZq =s4RD -----END PGP SIGNATURE----- --1ExXuwkHt/ur4MCY--