Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux(repost)

[Mathieu Desnoyers]

* Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) wrote:
> On Wed, Feb 11, 2009 at 11:08:24PM -0500, Mathieu Desnoyers wrote:
> > * Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) 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@xxxxxxxxxxxxxxxxxx) wrote:
> 
> [ . . . ]
> 
> > > 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.
> > > 
> > 
> > Great ! :) I added this version to the git repository, hopefully it's ok
> > with you ?
> 
> Works for me!
> 
> > > I will play with removing models of mb...
> > > 
> > 
> > OK, I see you already did..
> 
> I continued this, and surprisingly few are actually required, though
> I don't fully trust the modeling of removed memory barriers.
> 

On my side I cleaned up the code a lot, and actually added some barriers
;) Especially in the busy loops, where we expect the other thread's
value to change eventually between iterations. A smp_rmb() seems more
appropriate that barrier(). I also added a lot of comments about
barriers in the code, and made the reader side much easier to review.

Please feel free to comment on my added code comments.

Mathieu

> 							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 ->
> > > 		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();
> > > 	}
> > > }
> > 
> > 
> > -- 
> > Mathieu Desnoyers
> > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
> 
> _______________________________________________
> ltt-dev mailing list
> ltt-dev@xxxxxxxxxxxxxxxxxxxxx
> 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@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/