lkml.org 
[lkml]   [2009]   [Feb]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost)
* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Thu, Feb 12, 2009 at 12:47:07AM -0500, Mathieu Desnoyers wrote:
> > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > > 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?
> > >
> >
> > Not necessarily. If we don't have the proper memory barriers, we can
> > have the writer waiting on, say, parity 0 *before* it has performed the
> > parity switch. Therefore, even newly coming readers will add up to
> > parity 0.
>
> But the write that changes the parity will eventually make it out.
> OK, so your argument is that we at least need a compiler barrier?
>

It all depends on the assumptions we make. I am currently trying to
assume the most aggressive memory ordering I can think of. The model I
think about to represent it is that memory reads/writes are kept local
to the CPU until a memory barrier is encountered. I doubt it exists in
practice, bacause the CPU will eventually have to commit the information
to memory (hrm, are sure about this ?), but if we use that as a starting
point, I think this would cover the entire spectrum of possible memory
barriers issues. Also, it would be easy to verify formally. But maybe am
I going too far ?

> Regardless, please see attached for a modified version of the Promela
> model that fully models omitting out the memory barrier that my
> rcu_nest32.[hc] implementation omits. (It is possible to partially
> model removal of other memory barriers via #if 0, but to fully model
> would need to enumerate the permutations as shown on lines 231-257.)
>
> > In your model, this is not detected, because eventually all readers will
> > execute, and only then the writer will be able to update the data. But
> > in reality, if we run a very busy 4096-cores machines where there is
> > always at least one reader active, the the writer will be stuck forever,
> > and that's really bad.
>
> Assuming that the reordering is done by the CPU, the write will
> eventually get out -- it is stuck in (say) the store buffer, and the
> cache line will eventually arrive, and then the value will eventually
> be seen by the readers.

Do we have guarantees that the data *will necessarily* get out of the
cpu write buffer at some point ?

>
> We might need a -compiler- barrier, but then again, I am not sure that
> we are talking about the same memory barrier -- again, please see
> attached lines 231-257 to see which one that I eliminated.
>

As long as we don't have "progress" validation to check our model, the
fact that it passes the current test does not tell much.

> Also, the original model I sent out has a minor bug that prevents it
> from fully modeling the nested-read-side case. The patch below fixes this.
>

Ok, merging the fix, thanks,

Mathieu

> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>
> urcu.spin | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/formal-model/urcu.spin b/formal-model/urcu.spin
> index e5bfff3..611464b 100644
> --- a/formal-model/urcu.spin
> +++ b/formal-model/urcu.spin
> @@ -124,9 +124,13 @@ proctype urcu_reader()
> break;
> :: tmp < 4 && reader_progress[tmp] != 0 ->
> tmp = tmp + 1;
> - :: tmp >= 4 ->
> + :: tmp >= 4 &&
> + reader_progress[0] == reader_progress[3] ->
> done = 1;
> break;
> + :: tmp >= 4 &&
> + reader_progress[0] != reader_progress[3] ->
> + break;
> od;
> do
> :: tmp < 4 && reader_progress[tmp] == 0 ->

Content-Description: urcu_mbmin.spin
> /*
> * urcu_mbmin.spin: Promela code to validate urcu. See commit number
> * 3a9e6e9df706b8d39af94d2f027210e2e7d4106e of Mathieu Desnoyer's
> * git archive at git://lttng.org/userspace-rcu.git, but with
> * memory barriers removed.
> *
> * 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 &&
> reader_progress[0] == reader_progress[3] ->
> done = 1;
> break;
> :: tmp >= 4 &&
> reader_progress[0] != reader_progress[3] ->
> 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()
> {
> byte tmp;
>
> /* prior synchronize_rcu(), second counter flip. */
> need_mb = 1; /* mb() A */
> do
> :: need_mb == 1 -> skip;
> :: need_mb == 0 -> break;
> od;
> urcu_gp_ctr = urcu_gp_ctr + RCU_GP_CTR_BIT;
> need_mb = 1; /* mb() B */
> 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; /* mb() C absolutely required by analogy with G */
> do
> :: need_mb == 1 -> skip;
> :: need_mb == 0 -> break;
> od;
>
> /* Removal statement, e.g., list_del_rcu(). */
> removed = 1;
>
> /* current synchronize_rcu(), first counter flip. */
> need_mb = 1; /* mb() D suggested */
> do
> :: need_mb == 1 -> skip;
> :: need_mb == 0 -> break;
> od;
> urcu_gp_ctr = urcu_gp_ctr + RCU_GP_CTR_BIT;
> need_mb = 1; /* mb() E required if D not present */
> do
> :: need_mb == 1 -> skip;
> :: need_mb == 0 -> break;
> od;
>
> /* current synchronize_rcu(), first-flip check plus second flip. */
> if
> :: 1 ->
> 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;
> urcu_gp_ctr = urcu_gp_ctr + RCU_GP_CTR_BIT;
> :: 1 ->
> tmp = urcu_gp_ctr;
> urcu_gp_ctr = urcu_gp_ctr + RCU_GP_CTR_BIT;
> do
> :: 1 ->
> if
> :: (urcu_active_readers & RCU_GP_CTR_NEST_MASK) != 0 &&
> (urcu_active_readers & ~RCU_GP_CTR_NEST_MASK) !=
> (tmp & ~RCU_GP_CTR_NEST_MASK) ->
> skip;
> :: else -> break;
> fi;
> od;
> fi;
>
> /* current synchronize_rcu(), second counter flip check. */
> need_mb = 1; /* mb() F not required */
> 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; /* mb() G absolutely required */
> 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();
> }
> }



--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


\
 
 \ /
  Last update: 2009-02-12 19:43    [W:0.182 / U:0.216 seconds]
©2003-2014 Jasper Spaans. Advertise on this site