From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757891AbZBLCdW (ORCPT ); Wed, 11 Feb 2009 21:33:22 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752231AbZBLCdN (ORCPT ); Wed, 11 Feb 2009 21:33:13 -0500 Received: from e7.ny.us.ibm.com ([32.97.182.137]:51710 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751309AbZBLCdL (ORCPT ); Wed, 11 Feb 2009 21:33:11 -0500 Date: Wed, 11 Feb 2009 18:33:08 -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: <20090212023308.GA21157@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090210212833.GA27578@Krystal> <20090210222115.GN6742@linux.vnet.ibm.com> <20090211005701.GA550@Krystal> <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> MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="yrj/dFKFPuw6o+aM" Content-Disposition: inline In-Reply-To: <20090212003549.GU6694@linux.vnet.ibm.com> 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 --yrj/dFKFPuw6o+aM Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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: > > [ . . . ] > > > > > Hrm, let me present it in a different, more straightfoward way : > > > > > > > > In you Promela model (here : http://lkml.org/lkml/2009/2/10/419) > > > > > > > > There is a memory barrier here in the updater : > > > > > > > > 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; > > > > urcu_gp_ctr = urcu_gp_ctr + RCU_GP_CTR_BIT; > > > > > > I believe you were actually looking for a memory barrier here, not? > > > I do not believe that your urcu.c has a memory barrier here, please > > > see below. > > > > > > > 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; > > > > > > > > However, in your C code of nest_32.c, there is none. So it is at the > > > > very least an inconsistency between your code and your model. > > > > > > The urcu.c 3a9e6e9df706b8d39af94d2f027210e2e7d4106e lays out as follows: > > > > > > synchronize_rcu() > > > > > > switch_qparity() > > > > > > force_mb_all_threads() > > > > > > switch_next_urcu_qparity() [Just does counter flip] > > > > > > > Hrm... there would potentially be a missing mb() here. > > K, I added it to the model. > > > > wait_for_quiescent_state() > > > > > > Wait for all threads > > > > > > force_mb_all_threads() > > > My model does not represent this > > > memory barrier, because it seemed to > > > me that it was redundant with the > > > following one. > > > > > > > Yes, this one is redundant. > > I left it in for now... > > > > I added it, no effect. > > > > > > switch_qparity() > > > > > > force_mb_all_threads() > > > > > > switch_next_urcu_qparity() [Just does counter flip] > > > > > > > Same as above, potentially missing mb(). > > I added it to the model. > > > > wait_for_quiescent_state() > > > > > > Wait for all threads > > > > > > force_mb_all_threads() > > > > > > The rcu_nest32.c 6da793208a8f60ea41df60164ded85b4c5c5307d lays out as > > > follows: > > > > > > synchronize_rcu() > > > > > > flip_counter_and_wait() > > > > > > flips counter > > > > > > smp_mb(); > > > > > > Wait for threads > > > > > > > this is the point where I wonder if we should add a mb() to your code. > > Might well be, though I would argue for the very end, where I left out > the smp_mb(). I clearly need to make another Promela model for this > code, but we should probably focus on yours first, given that I don't > have any use cases for mine. > > > > flip_counter_and_wait() > > > > > > flips counter > > > > > > smp_mb(); > > > > > > Wait for threads > > And I really do have an unlock followed by an smp_mb() at this point. > > > > So, if I am reading the code correctly, I have memory barriers > > > everywhere you don't and vice versa. ;-) > > > > > > > Exactly. You have mb() between > > flips counter and (next) Wait for threads > > > > I have mb() between > > (previous) Wait for threads and flips counter > > > > Both might be required. Or none. :) > > Well, adding in the two to yours still gets Promela failures, please > see attached. Nothing quite like a multi-thousand step failure case, > I have to admit! ;-) > > > > The reason that I believe that I do not need a memory barrier between > > > the wait-for-threads and the subsequent flip is that the threads we > > > are waiting for have to have already committed to the earlier value of > > > the counter, and so changing the counter out of order has no effect. > > > > > > Does this make sense, or am I confused? > > > > So if we remove the mb() as in your code, between the flips counter and > > (next) Wait for thread, we are doing these operations in random order at > > the write site: > > I don't believe that I get to remove and mb()s from my code... > > > Sequence 1 - what we expect > > A.1 - flip counter > > A.2 - read counter > > B - read other threads urcu_active_readers > > > > So what happens if the CPU decides to reorder the unrelated > > operations? We get : > > > > Sequence 2 > > B - read other threads urcu_active_readers > > A.1 - flip counter > > A.2 - read counter > > > > Sequence 3 > > A.1 - flip counter > > A.2 - read counter > > B - read other threads urcu_active_readers > > > > Sequence 4 > > A.1 - flip counter > > B - read other threads urcu_active_readers > > A.2 - read counter > > > > > > Sequence 1, 3 and 4 are OK because the counter flip happens before we > > read other thread's urcu_active_readers counts. > > > > However, we have to consider Sequence 2 carefully, because we will read > > other threads uru_active_readers count before those readers see that we > > flipped the counter. > > > > The reader side does either : > > > > seq. 1 > > R.1 - read urcu_active_readers > > S.2 - read counter > > RS.2- write urcu_active_readers, depends on read counter and read > > urcu_active_readers > > > > (with R.1 and S.2 in random order) > > > > or > > > > seq. 2 > > R.1 - read urcu_active_readers > > R.2 - write urcu_active_readers, depends on read urcu_active_readers > > > > > > So we could have the following reader+writer sequence : > > > > Interleaved writer Sequence 2 and reader seq. 1. > > > > Reader: > > R.1 - read urcu_active_readers > > S.2 - read counter > > Writer: > > B - read other threads urcu_active_readers (there are none) > > A.1 - flip counter > > A.2 - read counter > > Reader: > > RS.2- write urcu_active_readers, depends on read counter and read > > urcu_active_readers > > > > Here, the reader would have updated its counter as belonging to the old > > q.s. period, but the writer will later wait for the new period. But > > given the writer will eventually do a second flip+wait, the reader in > > the other q.s. window will be caught by the second flip. > > > > Therefore, we could be tempted to think that those mb() could be > > unnecessary, which would lead to a scheme where urcu_active_readers and > > urcu_gp_ctr are done in a completely random order one vs the other. > > Let's see what it gives : > > > > synchronize_rcu() > > > > force_mb_all_threads() /* > > * Orders pointer publication and > > * (urcu_active_readers/urcu_gp_ctr accesses) > > */ > > switch_qparity() > > > > switch_next_urcu_qparity() [just does counter flip 0->1] > > > > wait_for_quiescent_state() > > > > wait for all threads in parity 0 > > > > switch_qparity() > > > > switch_next_urcu_qparity() [Just does counter flip 1->0] > > > > wait_for_quiescent_state() > > > > Wait for all threads in parity 1 > > > > force_mb_all_threads() /* > > * Orders > > * (urcu_active_readers/urcu_gp_ctr accesses) > > * and old data removal. > > */ > > > > > > > > *but* ! There is a reason why we don't want to do this. If > > > > switch_next_urcu_qparity() [Just does counter flip 1->0] > > > > happens before the end of the previous > > > > Wait for all threads in parity 0 > > > > We enter in a situation where all newly coming readers will see the > > parity bit as 0, although we are still waiting for that parity to end. > > We end up in a state when the writer can be blocked forever (no possible > > progress) if there are steadily readers subscribed for the data. > > > > Basically, to put it differently, we could simply remove the bit > > flipping from the writer and wait for *all* readers to exit their > > critical section (even the ones simply interested in the new pointer). > > But this shares the same problem the version above has, which is that we > > end up in a situation where the writer won't progress if there are > > always readers in a critical section. > > > > The same applies to > > > > switch_next_urcu_qparity() [Just does counter flip 0->1] > > > > wait for all threads in parity 0 > > > > If we don't put a mb() between those two (as I mistakenly did), we can > > end up waiting for readers in parity 0 while the parity bit wasn't > > flipped yet. oops. Same potential no-progress situation. > > > > The ordering of memory reads in the reader for > > urcu_active_readers/urcu_gp_ctr accesses does not seem to matter because > > the data contains information about which q.s. period parity it is in. > > In whichever order those variables are read seems to all work fine. > > > > In the end, it's to insure that the writer will always progress that we > > have to enforce smp_mb() between *all* switch_next_urcu_qparity and wait > > for threads. Mine and yours. > > > > Or maybe there is a detail I haven't correctly understood that insures > > this already without the mb() in your code ? > > > > > (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... Thanx, Paul --yrj/dFKFPuw6o+aM Content-Type: text/plain; charset=us-ascii Content-Description: urcu.spin Content-Disposition: attachment; filename="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 -> printf("urcu_gp_ctr=%x urcu_active_readers=%x\n", urcu_gp_ctr, urcu_active_readers); printf("urcu_gp_ctr&0x7f=%x urcu_active_readers&0x7f=%x\n", urcu_gp_ctr & ~RCU_GP_CTR_NEST_MASK, urcu_active_readers & ~RCU_GP_CTR_NEST_MASK); 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 -> printf("urcu_gp_ctr=%x urcu_active_readers=%x\n", urcu_gp_ctr, urcu_active_readers); printf("urcu_gp_ctr&0x7f=%x urcu_active_readers&0x7f=%x\n", urcu_gp_ctr & ~RCU_GP_CTR_NEST_MASK, urcu_active_readers & ~RCU_GP_CTR_NEST_MASK); 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(); } } --yrj/dFKFPuw6o+aM--