Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755063AbbFLMcS (ORCPT ); Fri, 12 Jun 2015 08:32:18 -0400 Received: from emvm-gh1-uea08.nsa.gov ([63.239.67.9]:61043 "EHLO emvm-gh1-uea08.nsa.gov" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754876AbbFLMcP (ORCPT ); Fri, 12 Jun 2015 08:32:15 -0400 X-TM-IMSS-Message-ID: Message-ID: <557AD10D.6060200@tycho.nsa.gov> Date: Fri, 12 Jun 2015 08:31:09 -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: 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> In-Reply-To: <557A7B91.4000502@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: 2741 Lines: 63 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. -- 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/