From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754991AbcK3Psu (ORCPT ); Wed, 30 Nov 2016 10:48:50 -0500 Received: from mx1.redhat.com ([209.132.183.28]:41840 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753631AbcK3Psf (ORCPT ); Wed, 30 Nov 2016 10:48:35 -0500 Date: Wed, 30 Nov 2016 13:48:10 -0200 From: Marcelo Tosatti To: "Yu, Fenghua" , Thomas Gleixner Cc: linux-kernel@vger.kernel.org Subject: [PATCH] intelrdt: resctrl: recommend locking for resctrlfs Message-ID: <20161130154809.GA27444@amt.cnet> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.21 (2010-09-15) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Wed, 30 Nov 2016 15:48:33 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org There is a locking problem between different applications reading/writing to resctrlfs directory at the same time (read the patch below for details). Suggest a standard locking scheme for applications to use. Signed-off-by: Marcelo Tosatti --- Documentation/x86/intel_rdt_ui.txt.orig 2016-11-30 13:40:33.080233101 -0200 +++ Documentation/x86/intel_rdt_ui.txt 2016-11-30 13:45:01.253703259 -0200 @@ -212,3 +212,30 @@ Finally we move core 4-7 over to the new kernel and the tasks running there get 50% of the cache. # echo C0 > p0/cpus + +4) Locking between applications + +The allocation of an exclusive reservation +of L3 cache involves: + + 1. read list of cbmmasks for each directory + 2. find a contiguous set of bits in the global CBM bitmask + that is clear in any of the directory cbmmasks + 3. create a new directory + 4. set the bits found in step 2 to the new directory "schemata" + file + +If two applications attempt to allocate space race with each other +(if two processes execute the steps above in a interlocked fashion), +they can end up using the same bits of CBMMASK, which renders the +reservations non-exclusive but shared. + +To coordinate creation of reservations on resctrl and avoid the problem +above, the following locking procedure is recommended: + +A) open /var/lock/resctrl/fs.lock with O_CREAT|O_EXCL. +B) if success, write pid of program accessing the directory + structure to this file. +C) read/write the directory structure. +D) remove file. +