Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932352AbbFKMjq (ORCPT ); Thu, 11 Jun 2015 08:39:46 -0400 Received: from emvm-gh1-uea09.nsa.gov ([63.239.67.10]:56098 "EHLO emvm-gh1-uea09.nsa.gov" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752886AbbFKMjj (ORCPT ); Thu, 11 Jun 2015 08:39:39 -0400 X-TM-IMSS-Message-ID: <849ba1b300108cef@nsa.gov> Message-ID: <55798149.5070404@tycho.nsa.gov> Date: Thu, 11 Jun 2015 08:38:33 -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: Waiman Long , Paul Moore , Eric Paris , James Morris , "Serge E. Hallyn" , selinux@tycho.nsa.gov CC: Scott J Norton , linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org, Douglas Hatch Subject: Re: [PATCH] selinux: reduce locking overhead in inode_free_security() References: <1433967446-44555-1-git-send-email-Waiman.Long@hp.com> In-Reply-To: <1433967446-44555-1-git-send-email-Waiman.Long@hp.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: 2495 Lines: 58 On 06/10/2015 04:17 PM, 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. We still need > to do the empty list test inside the lock for safety reason, but it > minimizes the chance of unnecessary spinlock contention. > > Signed-off-by: Waiman Long > --- > security/selinux/hooks.c | 17 +++++++++++++---- > 1 files changed, 13 insertions(+), 4 deletions(-) > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > index 7dade28..cd736c3 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)) > - list_del_init(&isec->list); > - spin_unlock(&sbsec->isec_lock); > + /* > + * 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. Lock taking can be slow > + * especially if the lock is being contended. We do, however, need > + * to recheck the list again before deleting it for safety. > + */ > + if (!list_empty(&isec->list)) { > + spin_lock(&sbsec->isec_lock); > + if (!list_empty(&isec->list)) > + list_del_init(&isec->list); > + spin_unlock(&sbsec->isec_lock); > + } > > /* > * The inode may still be referenced in a path walk and > Do we really need the second list_empty() test at all? Once removed, inode security structures are never re-added to the list. For comparison, inode_sb_list_del() only tests list_empty() outside the lock. -- 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/