Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933924AbWKWUnR (ORCPT ); Thu, 23 Nov 2006 15:43:17 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S933930AbWKWUnR (ORCPT ); Thu, 23 Nov 2006 15:43:17 -0500 Received: from pool-71-111-72-250.ptldor.dsl-w.verizon.net ([71.111.72.250]:60220 "EHLO IBM-8EC8B5596CA.beaverton.ibm.com") by vger.kernel.org with ESMTP id S933924AbWKWUnQ (ORCPT ); Thu, 23 Nov 2006 15:43:16 -0500 Date: Thu, 23 Nov 2006 12:40:54 -0800 From: "Paul E. McKenney" To: Oleg Nesterov Cc: Alan Stern , linux-kernel@vger.kernel.org Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync Message-ID: <20061123204054.GA4533@us.ibm.com> Reply-To: paulmck@us.ibm.com References: <20061117065128.GA5452@us.ibm.com> <20061117092925.GT7164@kernel.dk> <20061119190027.GA3676@oleg> <20061123145910.GA145@oleg> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20061123145910.GA145@oleg> User-Agent: Mutt/1.5.9i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2645 Lines: 93 On Thu, Nov 23, 2006 at 05:59:10PM +0300, Oleg Nesterov wrote: > (Sorry, responding to the wrong message) > > Paul E. McKenney wrote: > > > > I am concerned about this as well, and am beginning to suspect that I > > need to make a special-purpose primitive specifically for Jens that he > > can include with his code. > > How about this? For Jens, it might be OK. For general use, I believe that this has difficulties with the sequence of events I sent out on November 20th, see: http://marc.theaimsgroup.com/?l=linux-kernel&m=116397154808901&w=2 Might also be missing a few memory barriers, see below. > struct xxx_struct { > int completed; > atomic_t ctr[2]; > struct mutex mutex; > wait_queue_head_t wq; > }; > > void init_xxx_struct(struct xxx_struct *sp) > { > sp->completed = 0; > atomic_set(sp->ctr + 0, 1); // active > atomic_set(sp->ctr + 1, 0); // inactive > mutex_init(&sp->mutex); > init_waitqueue_head(&sp->wq); > } > > int xxx_read_lock(struct xxx_struct *sp) > { > for (;;) { > int idx = sp->completed & 0x1; > if (likely(atomic_inc_not_zero(sp->ctr + idx))) Need an after-atomic-inc memory barrier here? > return idx; > } > } > > void xxx_read_unlock(struct xxx_struct *sp, int idx) > { Need a before-atomic-dec memory barrier here? > if (unlikely(atomic_dec_and_test(sp->ctr + idx))) > wake_up(&sp->wq); > } > > void synchronize_xxx(struct xxx_struct *sp) > { > int idx; > > mutex_lock(&sp->mutex); > > idx = ++sp->completed & 0x1; > smp_mb__before_atomic_inc(); > atomic_inc(&sp->ctr + idx); > > idx = !idx; > if (!atomic_dec_and_test(&sp->ctr + idx)) > __wait_event(&sp->wq, !atomic_read(&sp->ctr + idx)); I don't understand why an unlucky sequence of events mightn't be able to hang this __wait_event(). Suppose we did the atomic_dec_and_test(), then some other CPU executed xxx_read_unlock(), finding no one to awaken, then we execute the __wait_event()? What am I missing here? > > mutex_unlock(&sp->mutex); > } > > Yes, cache thrashing... But I think this is hard to avoid if we want writer > to be fast. > > I do not claim this is the best solution, but for some reason I'd like to > suggest something that doesn't need synchronize_sched(). What do you think > about correctness at least? The general approach seems reasonable, but I do have the concerns above. Thanx, Paul - 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/