Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751323AbbETFJg (ORCPT ); Wed, 20 May 2015 01:09:36 -0400 Received: from cantor2.suse.de ([195.135.220.15]:39654 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750846AbbETFJf (ORCPT ); Wed, 20 May 2015 01:09:35 -0400 Date: Wed, 20 May 2015 15:09:19 +1000 From: NeilBrown To: "Paul E. McKenney" Cc: Steven Rostedt , Patrick Marlier , linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com, wangyun@linux.vnet.ibm.com Subject: Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage Message-ID: <20150520150919.7f5aa0e9@notabene.brown> In-Reply-To: <20150519220725.GA6776@linux.vnet.ibm.com> References: <20150512224603.GA4531@linux.vnet.ibm.com> <1431470787-4702-1-git-send-email-paulmck@linux.vnet.ibm.com> <1431470787-4702-3-git-send-email-paulmck@linux.vnet.ibm.com> <20150512223853.55e0ed90@grimm.local.home> <20150513125839.371ef677@notabene.brown> <5557819E.1060001@gmail.com> <20150518120647.0c3cecd8@notabene.brown> <20150518094321.2012a66a@gandalf.local.home> <20150519220725.GA6776@linux.vnet.ibm.com> X-Mailer: Claws Mail 3.10.1-162-g4d0ed6 (GTK+ 2.24.25; x86_64-suse-linux-gnu) MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; boundary="Sig_/ntDKUwQbw5ySmcebhba29Sq"; protocol="application/pgp-signature" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4181 Lines: 112 --Sig_/ntDKUwQbw5ySmcebhba29Sq Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable On Tue, 19 May 2015 15:07:25 -0700 "Paul E. McKenney" wrote: > The code in md probably needs to change in any case, as otherwise we are > invoking rcu_dereference_whatever() on a full struct list_head rather > than on a single pointer. Or am I missing something here? I think it would be rcu_dereference_whatever(&mddev->disks) I don't know what you mean by "on a full struct list_head", but there is nothing actually being dereferenced here - right? Just pointer arithmetic = on 'mddev'. I should probably just diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c index 2bc56e2a3526..b1d237bf8b3b 100644 --- a/drivers/md/bitmap.c +++ b/drivers/md/bitmap.c @@ -181,7 +181,7 @@ static struct md_rdev *next_active_rdev(struct md_rdev = *rdev, struct mddev *mdde rcu_read_lock(); if (rdev =3D=3D NULL) /* start at the beginning */ - rdev =3D list_entry_rcu(&mddev->disks, struct md_rdev, same_set); + rdev =3D list_entry(&mddev->disks, struct md_rdev, same_set); else { /* release the previous rdev and start from there. */ rdev_dec_pending(rdev, mddev); as there really are no RCU issues with getting that address. Maybe I should move it outside the rcu_read_lock() just to be blatant.... but that would make the code a lot more clumsy as the rdev_dec_pending must be inside the rcu_read_lock.. So this. Thanks, NeilBrown From: NeilBrown Date: Wed, 20 May 2015 15:05:09 +1000 Subject: [PATCH] md/bitmap: remove rcu annotation from pointer arithmetic. Evaluating "&mddev->disks" is simple pointer arithmetic, so it does not need 'rcu' annotations - no dereferencing is happening. Also enhance the comment to explain that 'rdev' in that case is not actually a pointer to an rdev. Reported-by: Patrick Marlier Signed-off-by: NeilBrown diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c index 2bc56e2a3526..135a0907e9de 100644 --- a/drivers/md/bitmap.c +++ b/drivers/md/bitmap.c @@ -177,11 +177,16 @@ static struct md_rdev *next_active_rdev(struct md_rde= v *rdev, struct mddev *mdde * nr_pending is 0 and In_sync is clear, the entries we return will * still be in the same position on the list when we re-enter * list_for_each_entry_continue_rcu. + * + * Note that if entered with 'rdev =3D=3D NULL' to start at the + * beginning, we temporarily assign 'rdev' to an address which + * isn't really an rdev, but which can be used by + * list_for_each_entry_continue_rcu() to find the first entry. */ rcu_read_lock(); if (rdev =3D=3D NULL) /* start at the beginning */ - rdev =3D list_entry_rcu(&mddev->disks, struct md_rdev, same_set); + rdev =3D list_entry(&mddev->disks, struct md_rdev, same_set); else { /* release the previous rdev and start from there. */ rdev_dec_pending(rdev, mddev); --Sig_/ntDKUwQbw5ySmcebhba29Sq Content-Type: application/pgp-signature Content-Description: OpenPGP digital signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQIVAwUBVVwW/znsnt1WYoG5AQJnzA//c5wgSwBVmDcDgRivvZOLumMyVnknzjnt yuGtTN0kKk4/4TbPbn0APnANQSGK7A/0M2FW9oBAm2tm5GrGhbpgjs2sdQDBhdl5 tEQbcZ2dah9JjSOvL4qA2ZNWKk1740yIf0zA7dfLi+gZNPuFUMnVHX9En1wcHngN 19RHHosalwNqg9v20KY/fWUgUvNOHH8UslJZBFdqUGlMAvgFYXKbKMJU82yfP7wj s8pvI2oVXfUOPkHuMP0uSb+nlD15cXhdqd7buioQpyxeHgYDS5AQWwhdcdzz4QJl 75rCv3gNfBoTHdJ410diTQDYOnxTGE1m8nqxGXWxy6dG3YYXtekTOLagLnwwT6xz sM4Ko/E3dtudIIj+H18PgEcR5/uCYSWWDvid8FhcUA8/A+gxN8R+SpDXmT+mZ8ON 69sAtqK7H702vQhwiliNq5rByEnOrbmWfpdx3uxoFahoT+8AjYFjKBpaIadt+GVb em7vrBCenUmRxzlxY5ZM9l8lS9cd08omtSVd+hftCF9yc8jKCLr8B/P0Z+sZfkNk B6EalJ7CO8z2IWNu+zTo9PZWhikTbmliFOJKUXAcu9HLrGzudUmZIEBPuy5U6Oi5 yibM7U9F0Y+3FOfdiod1AjC5F9dvijiEaWjJoeAs97GQ+JKxK9nmzpJaVDhG0wl4 YxKnUBFRABg= =KX4B -----END PGP SIGNATURE----- --Sig_/ntDKUwQbw5ySmcebhba29Sq-- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/