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    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


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