Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751400AbbFNEUD (ORCPT ); Sun, 14 Jun 2015 00:20:03 -0400 Received: from e28smtp09.in.ibm.com ([122.248.162.9]:50089 "EHLO e28smtp09.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750726AbbFNETx (ORCPT ); Sun, 14 Jun 2015 00:19:53 -0400 Message-ID: <557CFC87.2080700@linux.vnet.ibm.com> Date: Sun, 14 Jun 2015 09:31:11 +0530 From: Raghavendra K T Organization: IBM User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7 MIME-Version: 1.0 To: Waiman Long CC: Stephen Smalley , Paul Moore , Eric Paris , James Morris , "Serge E. Hallyn" , selinux@tycho.nsa.gov, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, Scott J Norton , Douglas Hatch Subject: Re: [PATCH v2] selinux: reduce locking overhead in inode_free_security() References: <1434058284-56634-1-git-send-email-Waiman.Long@hp.com> <557A7B91.4000502@linux.vnet.ibm.com> <557AD10D.6060200@tycho.nsa.gov> <557B5EBF.6010105@hp.com> In-Reply-To: <557B5EBF.6010105@hp.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15061404-0033-0000-0000-000006432051 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3675 Lines: 105 On 06/13/2015 04:05 AM, Waiman Long wrote: > On 06/12/2015 08:31 AM, Stephen Smalley wrote: >> On 06/12/2015 02:26 AM, Raghavendra K T wrote: >>> On 06/12/2015 03:01 AM, Waiman Long wrote: >>>> The inode_free_security() function just took the superblock's isec_lock >>>> before checking and trying to remove the inode security struct from the >>>> linked list. In many cases, the list was empty and so the lock taking >>>> is wasteful as no useful work is done. On multi-socket systems with >>>> a large number of CPUs, there can also be a fair amount of spinlock >>>> contention on the isec_lock if many tasks are exiting at the same time. >>>> >>>> This patch changes the code to check the state of the list first >>>> before taking the lock and attempting to dequeue it. As this function >>>> is called indirectly from __destroy_inode(), there can't be another >>>> instance of inode_free_security() running on the same inode. >>>> >>>> Signed-off-by: Waiman Long >>>> --- >>>> security/selinux/hooks.c | 15 ++++++++++++--- >>>> 1 files changed, 12 insertions(+), 3 deletions(-) >>>> >>>> v1->v2: >>>> - Take out the second list_empty() test inside the lock. >>>> >>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c >>>> index 7dade28..e5cdad7 100644 >>>> --- a/security/selinux/hooks.c >>>> +++ b/security/selinux/hooks.c >>>> @@ -254,10 +254,19 @@ static void inode_free_security(struct inode >>>> *inode) >>>> struct inode_security_struct *isec = inode->i_security; >>>> struct superblock_security_struct *sbsec = >>>> inode->i_sb->s_security; >>>> >>>> - spin_lock(&sbsec->isec_lock); >>>> - if (!list_empty(&isec->list)) >>>> + /* >>>> + * As not all inode security structures are in a list, we check >>>> for >>>> + * empty list outside of the lock to make sure that we won't waste >>>> + * time taking a lock doing nothing. As inode_free_security() is >>>> + * being called indirectly from __destroy_inode(), there is no way >>>> + * there can be two or more concurrent calls. So doing the >>>> list_empty() >>>> + * test outside the loop should be safe. >>>> + */ >>>> + if (!list_empty(&isec->list)) { >>>> + spin_lock(&sbsec->isec_lock); >>>> list_del_init(&isec->list); >>> Stupid question, >>> >>> I need to take a look at list_del_init() code, but it can so happen that >>> if !list_empty() check could happen simultaneously, then serially two >>> list_del_init() can happen. >>> >>> is that not a problem()? >> Hmm...I suppose that's possible (sb_finish_set_opts and >> inode_free_security could both perform the list_del_init). Ok, we'll >> stay with the first version. >> > > Actually, list_del_init() can be applied twice with no harm being done. > The first list_del_init() will set list-> next = list->prev = list. The > second one will do the same thing and so it should be safe. > Waiman, I do not think it is just about list_del_init() twice what if CPU1 CPU2 CPU3 !list_empty() !list_empty() lock list_del_init() unlock list_add() lock list_del_init unlock But this is valid only if list_add() is possible after first list_del_init. I need to see code though. OR am I missing something? -- 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/