Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755579AbbFONju (ORCPT ); Mon, 15 Jun 2015 09:39:50 -0400 Received: from emvm-gh1-uea09.nsa.gov ([63.239.67.10]:56989 "EHLO emvm-gh1-uea09.nsa.gov" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754074AbbFONjm (ORCPT ); Mon, 15 Jun 2015 09:39:42 -0400 X-TM-IMSS-Message-ID: <996be8af00012b2a@nsa.gov> Message-ID: <557ED55B.4010604@tycho.nsa.gov> Date: Mon, 15 Jun 2015 09:38:35 -0400 From: Stephen Smalley Organization: National Security Agency User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Raghavendra K T , Waiman Long CC: James Morris , Scott J Norton , linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, Douglas Hatch , selinux@tycho.nsa.gov 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> <557CFC87.2080700@linux.vnet.ibm.com> In-Reply-To: <557CFC87.2080700@linux.vnet.ibm.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3937 Lines: 103 On 06/14/2015 12:01 AM, Raghavendra K T wrote: > 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? That should never be possible AFAICS. So I guess the second version is also safe. -- 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/