Messages in this thread | | | Date | Thu, 12 Feb 2009 13:40:30 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost) |
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* 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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