[ltt-dev] [PATCH URCU formal] Add liveness checks to user-level RCU model.
Paul E. McKenney
paulmck at linux.vnet.ibm.com
Fri Feb 20 13:44:59 EST 2009
By the way, this model does -not- cover Mathieu's scenario where a very
large number of reading processes continually arrive using an obsolete
value of the urcu_gp_ctr variable. Something about having finite memory
on all the machines I have access to. ;-)
So, here is an informal outline of why forward progress is guaranteed
in this situation:
o We assume that if one CPU makes a change to a shared variable,
all other CPUs see the change in a finite period of time.
[CPUs like the semi-SMP Blackfins will need to make this be the
case in software. If you cannot make this assumption, you cannot
prove that -any- non-trivial SMP algorithm will complete.]
o We assume that any given CPU executes code at a finite rate.
Of course, I would be very interested in hearing of any real
hardware that violates this assumption. ;-)
o We assume that all RCU read-side critical sections complete
in finite time (which can be violated if there are CPU-bound
real-time-priority processes unless you have priority boosting,
a scenario I am excluding -- but if this is a real problem,
we need to add priority boosting).
o Then there must be a finite number of threads that have observed
the old value of urcu_gp_ctr.
o Any subsequent RCU read-side critical sections will use the new
value of urcu_gp_ctr.
o The total time consumed by these threads' RCU read-side critical
sections will then be finite.
o The updater will therefore eventually observe all the threads
that used the old value of urcu_gp_ctr to have exited all of
their RCU read-side critical sections.
So given the three assumptions above, forward progress is guaranteed.
Note that although we don't explicitly assume a finite number of threads,
the finite-CPU-speed assumption makes it difficult to see how the updater
scans the per-thread variables in finite time. ;-)
Thanx, Paul
On Fri, Feb 20, 2009 at 09:39:27AM -0800, Paul E. McKenney wrote:
> Break all potentially infinite loops in both urcu_reader() and
> urcu_updater(), ensure that urcu_reader() will process any memory barriers
> that urcu_updater() might issue, and formulate a "never" claim that checks
> to make sure that if either urcu_reader() or urcu_updater() completes,
> then the other will eventually also complete. Since urcu_reader()
> now has a finite number of steps, it must eventually complete.
>
> Also replace the code at the end of urcu_reader() that previously absorbed
> late memory-barrier requests from urcu_updater with code in urcu_writer()
> that checks to see if urcu_reader() has completed.
>
> Signed-off-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
> ---
> formal-model/urcu.sh | 26 ++++++++++-
> formal-model/urcu.spin | 113 ++++++++++++++++++++++-------------------------
> 2 files changed, 77 insertions(+), 62 deletions(-)
>
> diff --git a/formal-model/urcu.sh b/formal-model/urcu.sh
> index b76c764..5e525ec 100644
> --- a/formal-model/urcu.sh
> +++ b/formal-model/urcu.sh
> @@ -1,3 +1,25 @@
> +#!/bin/sh
> +#
> +# Compiles and runs the urcu.spin Promela model.
> +#
> +# 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) IBM Corporation, 2009
> +#
> +# Authors: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
> +
> spin -a urcu.spin
> -cc -DSAFETY -o pan pan.c
> -./pan
> +cc -o pan pan.c
> +./pan -a
> diff --git a/formal-model/urcu.spin b/formal-model/urcu.spin
> index 3e18457..cf1f670 100644
> --- a/formal-model/urcu.spin
> +++ b/formal-model/urcu.spin
> @@ -40,21 +40,28 @@ bit updater_done = 0;
> byte urcu_gp_ctr = 1;
> byte urcu_active_readers = 0;
>
> +/* must be at least the number of smp_mb() statements in urcu_updater(). */
> +#define N_SMP_MB 9
> +
> /* Model the RCU read-side critical section. */
>
> proctype urcu_reader()
> {
> bit done = 0;
> + byte i;
> bit mbok;
> byte tmp;
> byte tmp_removed;
> byte tmp_free;
>
> /* Absorb any early requests for memory barriers. */
> + i = 0;
> do
> - :: need_mb == 1 ->
> + :: i < N_SMP_MB && need_mb == 1 ->
> need_mb = 0;
> - :: !updater_done -> skip;
> + i++;
> + :: i < N_SMP_MB ->
> + i++;
> :: 1 -> break;
> od;
>
> @@ -91,12 +98,17 @@ proctype urcu_reader()
> if
> :: (tmp & RCU_GP_CTR_NEST_MASK) == 0 ->
> tmp = urcu_gp_ctr;
> + i = 0;
> do
> - :: (reader_progress[1] +
> + :: i < N_SMP_MB &&
> + (reader_progress[1] +
> reader_progress[2] +
> - reader_progress[3] == 0) && need_mb == 1 ->
> + reader_progress[3] == 0) &&
> + need_mb == 1 ->
> need_mb = 0;
> - :: !updater_done -> skip;
> + i++;
> + :: i < N_SMP_MB ->
> + i++;
> :: 1 -> break;
> od;
> urcu_active_readers = tmp;
> @@ -151,10 +163,13 @@ proctype urcu_reader()
> if
> :: mbok == 1 ->
> /* We get here if mb processing is safe. */
> + i = 0;
> do
> - :: need_mb == 1 ->
> + :: i < N_SMP_MB && need_mb == 1 ->
> need_mb = 0;
> - :: !updater_done -> skip;
> + i++;
> + :: i < N_SMP_MB ->
> + i++;
> :: 1 -> break;
> od;
> :: else -> skip;
> @@ -169,36 +184,30 @@ proctype urcu_reader()
> :: else -> skip;
> fi
> od;
> +
> + /* Make sure any concurrent grace period was sufficiently long. */
> assert((tmp_free == 0) || (tmp_removed == 1));
>
> /* Reader has completed. */
> reader_done = 1;
> -
> - /* Process any late-arriving memory-barrier requests. */
> - do
> - :: need_mb == 1 ->
> - need_mb = 0;
> - :: !updater_done -> skip;
> - :: 1 -> break;
> - od;
> }
>
> +#define smp_mb() \
> + need_mb = 1; \
> + do \
> + :: !reader_done && need_mb == 1 -> skip; \
> + :: reader_done -> break; \
> + :: need_mb == 0 -> break; \
> + od
> +
> /* Model the RCU update process. */
>
> proctype urcu_updater()
> {
> /* prior synchronize_rcu(), second counter flip. */
> - need_mb = 1;
> - do
> - :: need_mb == 1 -> skip;
> - :: need_mb == 0 -> break;
> - od;
> + smp_mb();
> urcu_gp_ctr = urcu_gp_ctr + RCU_GP_CTR_BIT;
> - need_mb = 1;
> - do
> - :: need_mb == 1 -> skip;
> - :: need_mb == 0 -> break;
> - od;
> + smp_mb();
> do
> :: 1 ->
> if
> @@ -209,27 +218,15 @@ proctype urcu_updater()
> :: else -> break;
> fi;
> od;
> - need_mb = 1;
> - do
> - :: need_mb == 1 -> skip;
> - :: need_mb == 0 -> break;
> - od;
> + smp_mb();
>
> /* Removal statement, e.g., list_del_rcu(). */
> removed = 1;
>
> /* current synchronize_rcu(), first counter flip. */
> - need_mb = 1;
> - do
> - :: need_mb == 1 -> skip;
> - :: need_mb == 0 -> break;
> - od;
> + smp_mb();
> urcu_gp_ctr = urcu_gp_ctr + RCU_GP_CTR_BIT;
> - need_mb = 1;
> - do
> - :: need_mb == 1 -> skip;
> - :: need_mb == 0 -> break;
> - od;
> + smp_mb();
> do
> :: 1 ->
> if
> @@ -240,24 +237,12 @@ proctype urcu_updater()
> :: else -> break;
> fi;
> od;
> - need_mb = 1;
> - do
> - :: need_mb == 1 -> skip;
> - :: need_mb == 0 -> break;
> - od;
> + smp_mb();
>
> /* current synchronize_rcu(), second counter flip. */
> - need_mb = 1;
> - do
> - :: need_mb == 1 -> skip;
> - :: need_mb == 0 -> break;
> - od;
> + smp_mb();
> urcu_gp_ctr = urcu_gp_ctr + RCU_GP_CTR_BIT;
> - need_mb = 1;
> - do
> - :: need_mb == 1 -> skip;
> - :: need_mb == 0 -> break;
> - od;
> + smp_mb();
> do
> :: 1 ->
> if
> @@ -268,11 +253,7 @@ proctype urcu_updater()
> :: else -> break;
> fi;
> od;
> - need_mb = 1;
> - do
> - :: need_mb == 1 -> skip;
> - :: need_mb == 0 -> break;
> - od;
> + smp_mb();
>
> /* free-up step, e.g., kfree(). */
> free = 1;
> @@ -299,3 +280,15 @@ init {
> run urcu_updater();
> }
> }
> +
> +/* Require that both reader and updater eventually get done. */
> +
> +never {
> + do
> + :: skip;
> + :: reader_done != 0 || updater_done != 0 -> break;
> + od;
> +accept: do
> + :: reader_done == 0 || updater_done == 0;
> + od;
> +}
> --
> 1.5.2.5
>
More information about the lttng-dev
mailing list