From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752270AbZBLFJW (ORCPT ); Thu, 12 Feb 2009 00:09:22 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750771AbZBLFJN (ORCPT ); Thu, 12 Feb 2009 00:09:13 -0500 Received: from e4.ny.us.ibm.com ([32.97.182.144]:33206 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750721AbZBLFJM (ORCPT ); Thu, 12 Feb 2009 00:09:12 -0500 Date: Wed, 11 Feb 2009 21:09:09 -0800 From: "Paul E. McKenney" To: Mathieu Desnoyers Cc: ltt-dev@lists.casi.polymtl.ca, linux-kernel@vger.kernel.org Subject: Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost) Message-ID: <20090212050909.GB8317@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090211052828.GQ6742@linux.vnet.ibm.com> <20090211063520.GE8132@Krystal> <20090211153246.GA6694@linux.vnet.ibm.com> <20090211185203.GA29852@Krystal> <20090211200903.GG6694@linux.vnet.ibm.com> <20090211214258.GA32407@Krystal> <20090212003549.GU6694@linux.vnet.ibm.com> <20090212023308.GA21157@linux.vnet.ibm.com> <20090212023707.GA21193@linux.vnet.ibm.com> <20090212041044.GA12612@Krystal> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090212041044.GA12612@Krystal> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 11, 2009 at 11:10:44PM -0500, Mathieu Desnoyers wrote: > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > On Wed, Feb 11, 2009 at 06:33:08PM -0800, Paul E. McKenney wrote: > > > On Wed, Feb 11, 2009 at 04:35:49PM -0800, Paul E. McKenney wrote: > > > > On Wed, Feb 11, 2009 at 04:42:58PM -0500, Mathieu Desnoyers wrote: > > > > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > > > [ . . . ] > > > > > > > > (BTW, I do not trust my model yet, as it currently cannot detect the > > > > > > failure case I pointed out earlier. :-/ Here and I thought that the > > > > > > point of such models was to detect additional failure cases!!!) > > > > > > > > > > > > > > > > Yes, I'll have to dig deeper into it. > > > > > > > > Well, as I said, I attached the current model and the error trail. > > > > > > And I had bugs in my model that allowed the rcu_read_lock() model > > > to nest indefinitely, which overflowed into the top bit, messing > > > things up. :-/ > > > > > > Attached is a fixed model. This model validates correctly (woo-hoo!). > > > Even better, gives the expected error if you comment out line 180 and > > > uncomment line 213, this latter corresponding to the error case I called > > > out a few days ago. > > > > > > I will play with removing models of mb... > > > > And commenting out the models of mb between the counter flips and the > > test for readers still passes validation, as expected, and as shown in > > the attached Promela code. > > > > Hrm, in the email I sent you about the memory barrier, I said that it > would not make the algorithm incorrect, but that it would cause > situations where it would be impossible for the writer to do any > progress as long as there are readers active. I think we would have to > enhance the model or at least express this through some LTL statement to > validate this specific behavior. But if the writer fails to make progress, then the counter remains at a given value, which causes readers to drain, which allows the writer to eventually make progress again. Right? Thanx, Paul > Mathieu > > > Thanx, Paul > > Content-Description: urcu.spin > > /* > > * urcu.spin: Promela code to validate urcu. See commit number > > * 3a9e6e9df706b8d39af94d2f027210e2e7d4106e of Mathieu Desnoyer's > > * git archive at git://lttng.org/userspace-rcu.git > > * > > * This program is free software; you can redistribute it and/or modify > > * it under the terms of the GNU General Public License as published by > > * the Free Software Foundation; either version 2 of the License, or > > * (at your option) any later version. > > * > > * This program is distributed in the hope that it will be useful, > > * but WITHOUT ANY WARRANTY; without even the implied warranty of > > * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the > > * GNU General Public License for more details. > > * > > * You should have received a copy of the GNU General Public License > > * along with this program; if not, write to the Free Software > > * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. > > * > > * Copyright (c) 2009 Paul E. McKenney, IBM Corporation. > > */ > > > > /* Promela validation variables. */ > > > > bit removed = 0; /* Has RCU removal happened, e.g., list_del_rcu()? */ > > bit free = 0; /* Has RCU reclamation happened, e.g., kfree()? */ > > bit need_mb = 0; /* =1 says need reader mb, =0 for reader response. */ > > byte reader_progress[4]; > > /* Count of read-side statement executions. */ > > > > /* urcu definitions and variables, taken straight from the algorithm. */ > > > > #define RCU_GP_CTR_BIT (1 << 7) > > #define RCU_GP_CTR_NEST_MASK (RCU_GP_CTR_BIT - 1) > > > > byte urcu_gp_ctr = 1; > > byte urcu_active_readers = 0; > > > > /* Model the RCU read-side critical section. */ > > > > proctype urcu_reader() > > { > > bit done = 0; > > bit mbok; > > byte tmp; > > byte tmp_removed; > > byte tmp_free; > > > > /* Absorb any early requests for memory barriers. */ > > do > > :: need_mb == 1 -> > > need_mb = 0; > > :: 1 -> skip; > > :: 1 -> break; > > od; > > > > /* > > * Each pass through this loop executes one read-side statement > > * from the following code fragment: > > * > > * rcu_read_lock(); [0a] > > * rcu_read_lock(); [0b] > > * p = rcu_dereference(global_p); [1] > > * x = p->data; [2] > > * rcu_read_unlock(); [3b] > > * rcu_read_unlock(); [3a] > > * > > * Because we are modeling a weak-memory machine, these statements > > * can be seen in any order, the only restriction being that > > * rcu_read_unlock() cannot precede the corresponding rcu_read_lock(). > > * The placement of the inner rcu_read_lock() and rcu_read_unlock() > > * is non-deterministic, the above is but one possible placement. > > * Intestingly enough, this model validates all possible placements > > * of the inner rcu_read_lock() and rcu_read_unlock() statements, > > * with the only constraint being that the rcu_read_lock() must > > * precede the rcu_read_unlock(). > > * > > * We also respond to memory-barrier requests, but only if our > > * execution happens to be ordered. If the current state is > > * misordered, we ignore memory-barrier requests. > > */ > > do > > :: 1 -> > > if > > :: reader_progress[0] < 2 -> /* [0a and 0b] */ > > tmp = urcu_active_readers; > > if > > :: (tmp & RCU_GP_CTR_NEST_MASK) == 0 -> > > tmp = urcu_gp_ctr; > > do > > :: (reader_progress[1] + > > reader_progress[2] + > > reader_progress[3] == 0) && need_mb == 1 -> > > need_mb = 0; > > :: 1 -> skip; > > :: 1 -> break; > > od; > > urcu_active_readers = tmp; > > :: else -> > > urcu_active_readers = tmp + 1; > > fi; > > reader_progress[0] = reader_progress[0] + 1; > > :: reader_progress[1] == 0 -> /* [1] */ > > tmp_removed = removed; > > reader_progress[1] = 1; > > :: reader_progress[2] == 0 -> /* [2] */ > > tmp_free = free; > > reader_progress[2] = 1; > > :: ((reader_progress[0] > reader_progress[3]) && > > (reader_progress[3] < 2)) -> /* [3a and 3b] */ > > tmp = urcu_active_readers - 1; > > urcu_active_readers = tmp; > > reader_progress[3] = reader_progress[3] + 1; > > :: else -> break; > > fi; > > > > /* Process memory-barrier requests, if it is safe to do so. */ > > atomic { > > mbok = 0; > > tmp = 0; > > do > > :: tmp < 4 && reader_progress[tmp] == 0 -> > > tmp = tmp + 1; > > break; > > :: tmp < 4 && reader_progress[tmp] != 0 -> > > tmp = tmp + 1; > > :: tmp >= 4 -> > > done = 1; > > break; > > od; > > do > > :: tmp < 4 && reader_progress[tmp] == 0 -> > > tmp = tmp + 1; > > :: tmp < 4 && reader_progress[tmp] != 0 -> > > break; > > :: tmp >= 4 -> > > mbok = 1; > > break; > > od > > > > } > > > > if > > :: mbok == 1 -> > > /* We get here if mb processing is safe. */ > > do > > :: need_mb == 1 -> > > need_mb = 0; > > :: 1 -> skip; > > :: 1 -> break; > > od; > > :: else -> skip; > > fi; > > > > /* > > * Check to see if we have modeled the entire RCU read-side > > * critical section, and leave if so. > > */ > > if > > :: done == 1 -> break; > > :: else -> skip; > > fi > > od; > > assert((tmp_free == 0) || (tmp_removed == 1)); > > > > /* Process any late-arriving memory-barrier requests. */ > > do > > :: need_mb == 1 -> > > need_mb = 0; > > :: 1 -> skip; > > :: 1 -> break; > > od; > > } > > > > /* Model the RCU update process. */ > > > > proctype urcu_updater() > > { > > /* Removal statement, e.g., list_del_rcu(). */ > > removed = 1; > > > > /* synchronize_rcu(), first counter flip. */ > > need_mb = 1; > > do > > :: need_mb == 1 -> skip; > > :: need_mb == 0 -> break; > > od; > > urcu_gp_ctr = urcu_gp_ctr + RCU_GP_CTR_BIT; > > /* need_mb = 1; > > do > > :: need_mb == 1 -> skip; > > :: need_mb == 0 -> break; > > od; */ > > do > > :: 1 -> > > if > > :: (urcu_active_readers & RCU_GP_CTR_NEST_MASK) != 0 && > > (urcu_active_readers & ~RCU_GP_CTR_NEST_MASK) != > > (urcu_gp_ctr & ~RCU_GP_CTR_NEST_MASK) -> > > skip; > > :: else -> break; > > fi > > od; > > need_mb = 1; > > do > > :: need_mb == 1 -> skip; > > :: need_mb == 0 -> break; > > od; > > > > /* Erroneous removal statement, e.g., list_del_rcu(). */ > > /* removed = 1; */ > > > > /* synchronize_rcu(), second counter flip. */ > > need_mb = 1; > > do > > :: need_mb == 1 -> skip; > > :: need_mb == 0 -> break; > > od; > > urcu_gp_ctr = urcu_gp_ctr + RCU_GP_CTR_BIT; > > /* need_mb = 1; > > do > > :: need_mb == 1 -> skip; > > :: need_mb == 0 -> break; > > od; */ > > do > > :: 1 -> > > if > > :: (urcu_active_readers & RCU_GP_CTR_NEST_MASK) != 0 && > > (urcu_active_readers & ~RCU_GP_CTR_NEST_MASK) != > > (urcu_gp_ctr & ~RCU_GP_CTR_NEST_MASK) -> > > skip; > > :: else -> break; > > fi; > > od; > > need_mb = 1; > > do > > :: need_mb == 1 -> skip; > > :: need_mb == 0 -> break; > > od; > > > > /* free-up step, e.g., kfree(). */ > > free = 1; > > } > > > > /* > > * Initialize the array, spawn a reader and an updater. Because readers > > * are independent of each other, only one reader is needed. > > */ > > > > init { > > atomic { > > reader_progress[0] = 0; > > reader_progress[1] = 0; > > reader_progress[2] = 0; > > reader_progress[3] = 0; > > run urcu_reader(); > > run urcu_updater(); > > } > > } > > > _______________________________________________ > > ltt-dev mailing list > > ltt-dev@lists.casi.polymtl.ca > > http://lists.casi.polymtl.ca/cgi-bin/mailman/listinfo/ltt-dev > > > -- > Mathieu Desnoyers > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 > -- > 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/