Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754942Ab0HDEeK (ORCPT ); Wed, 4 Aug 2010 00:34:10 -0400 Received: from SMTP.ANDREW.CMU.EDU ([128.2.11.96]:36398 "EHLO smtp.andrew.cmu.edu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753249Ab0HDEeI (ORCPT ); Wed, 4 Aug 2010 00:34:08 -0400 Date: Wed, 4 Aug 2010 00:33:28 -0400 From: Ben Blum To: Paul Menage Cc: Ben Blum , Andrew Morton , linux-kernel@vger.kernel.org, containers@lists.linux-foundation.org, ebiederm@xmission.com, lizf@cn.fujitsu.com, matthltc@us.ibm.com, oleg@redhat.com Subject: Re: [PATCH v4 1/2] cgroups: read-write lock CLONE_THREAD forking per threadgroup Message-ID: <20100804043328.GB11950@ghc17.ghc.andrew.cmu.edu> References: <20100730235649.GA22644@ghc17.ghc.andrew.cmu.edu> <20100730235754.GB22644@ghc17.ghc.andrew.cmu.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.20 (2009-06-14) X-PMX-Version: 5.5.9.388399, Antispam-Engine: 2.7.2.376379, Antispam-Data: 2010.4.9.4220 X-SMTP-Spam-Clean: 8% ( BODY_SIZE_1600_1699 0, BODY_SIZE_2000_LESS 0, BODY_SIZE_5000_LESS 0, BODY_SIZE_7000_LESS 0, __BOUNCE_CHALLENGE_SUBJ 0, __BOUNCE_NDR_SUBJ_EXEMPT 0, __CD 0, __CT 0, __CT_TEXT_PLAIN 0, __HAS_MSGID 0, __MIME_TEXT_ONLY 0, __MIME_VERSION 0, __SANE_MSGID 0, __TO_MALFORMED_2 0, __URI_NO_PATH 0, __URI_NO_WWW 0, __USER_AGENT 0) X-SMTP-Spam-Score: 8% Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1843 Lines: 44 On Tue, Aug 03, 2010 at 08:44:01PM -0700, Paul Menage wrote: > On Fri, Jul 30, 2010 at 4:57 PM, Ben Blum wrote: > > + * The threadgroup_fork_lock prevents threads from forking with > > + * CLONE_THREAD while held for writing. Use this for fork-sensitive > > + * threadgroup-wide operations. It's taken for reading in fork.c in > > + * copy_process(). > > + * Currently only needed write-side by cgroups. > > + */ > > + struct rw_semaphore threadgroup_fork_lock; > > +#endif > > I'm not sure how best to word this comment, but I'd prefer something like: > > "The threadgroup_fork_lock is taken in read mode during a CLONE_THREAD > fork operation; taking it in write mode prevents the owning > threadgroup from adding any new threads and thus allows you to > synchronize against the addition of unseen threads when performing > threadgroup-wide operations. New-process forks (without CLONE_THREAD) > are not affected." That sounds good. > As far as the #ifdef mess goes, it's true that some people don't have > CONFIG_CGROUPS defined. I'd imagine that these are likely to be > embedded systems with a fairly small number of processes and threads > per process. Are there really any such platforms where the cost of a > single extra rwsem per process is going to make a difference either in > terms of memory or lock contention? I think you should consider making > these additions unconditional. That's certainly an option, but I think it would be clean enough to put static inline functions just under the signal_struct definition. Thoughts? > > Paul > -- Ben -- 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/