From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759968AbcLBKuj (ORCPT ); Fri, 2 Dec 2016 05:50:39 -0500 Received: from mx1.redhat.com ([209.132.183.28]:45566 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751786AbcLBKuh (ORCPT ); Fri, 2 Dec 2016 05:50:37 -0500 Date: Thu, 1 Dec 2016 12:55:44 -0200 From: Marcelo Tosatti To: Fenghua Yu Cc: Thomas Gleixner , linux-kernel@vger.kernel.org Subject: [PATCH v2] intelrdt: resctrl: recommend locking for resctrlfs Message-ID: <20161201145541.GA19232@amt.cnet> References: <20161130154809.GA27444@amt.cnet> <20161130220530.GG35583@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161130220530.GG35583@linux.intel.com> 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]); Fri, 02 Dec 2016 10:50:37 +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 --- v2: Improve commentary about generality of locking problem ("Yu, Fenghua") Proper patch -p level (Thomas Gleixner) diff --git a/Documentation/x86/intel_rdt_ui.txt b/Documentation/x86/intel_rdt_ui.txt index d918d26..14b3214 100644 --- a/Documentation/x86/intel_rdt_ui.txt +++ b/Documentation/x86/intel_rdt_ui.txt @@ -212,3 +212,33 @@ Finally we move core 4-7 over to the new group and make sure that the kernel and the tasks running there get 50% of the cache. # echo C0 > p0/cpus + +4) Locking between applications + +Certain operations on the resctrl filesystem, composed of +read / writes to multiple files, must be atomic. + +As an example, 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 attempting 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 atomic operations 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. +