Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 24 Nov 2014 14:36:18 -0800 | Subject | Re: [PATCH v4 2/5] x86, traps: Track entry into and exit from IST context |
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On Mon, Nov 24, 2014 at 2:34 PM, Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > On Mon, Nov 24, 2014 at 01:35:01PM -0800, Paul E. McKenney wrote: >> On Mon, Nov 24, 2014 at 01:02:51PM -0800, Andy Lutomirski wrote: >> > On Mon, Nov 24, 2014 at 12:54 PM, Paul E. McKenney >> > <paulmck@linux.vnet.ibm.com> wrote: >> > > On Mon, Nov 24, 2014 at 12:22:13PM -0800, Andy Lutomirski wrote: >> > >> On Sat, Nov 22, 2014 at 3:41 PM, Paul E. McKenney >> > >> <paulmck@linux.vnet.ibm.com> wrote: >> > >> > On Fri, Nov 21, 2014 at 09:53:29PM -0800, Andy Lutomirski wrote: >> > >> >> On Fri, Nov 21, 2014 at 8:20 PM, Paul E. McKenney >> > >> >> <paulmck@linux.vnet.ibm.com> wrote: >> > >> >> > On Fri, Nov 21, 2014 at 06:00:14PM -0800, Andy Lutomirski wrote: >> > >> >> >> On Fri, Nov 21, 2014 at 3:38 PM, Paul E. McKenney >> > >> >> >> <paulmck@linux.vnet.ibm.com> wrote: >> > >> >> > >> > Returning state sounds like a bad idea, if we can reasonably avoid it. >> > >> >> > >> I agree, except that we already do it for exception_enter(), etc. But >> > >> yes, changing fewer things is nice. >> > >> >> > >> > >> > >> > And I think I finally see what you are pointing out about my code: If >> > >> > another NMI comes in between the time I increment ->dynticks_nmi_nesting >> > >> > and the time I atomically increment ->dynticks, the nested NMI handler >> > >> > will incorrectly believe that RCU is already paying attention to this CPU. >> > >> > Which would indeed not be at all good, so good catch!!! >> > >> > >> > >> >> Otherwise, I think that there may need to be enough state somewhere so >> > >> >> that the outermost nested rcu_nmi_enter knows whether to increment >> > >> >> dynticks. For example, dynticks_nmi_nesting could store the nesting >> > >> >> count * 2 - (1 if the outermost nested user needs to increment >> > >> >> dynticks). Something like: >> > >> >> >> > >> >> void rcu_nmi_enter(void) >> > >> >> { >> > >> >> /* Be very careful -- this function may be called reentrently on the >> > >> >> same CPU. */ >> > >> >> atomically: increment dynticks if it's even. >> > >> >> >> > >> >> /* If an rcu_nmi_enter/rcu_nmi_exit pair happens here, then it will not change >> > >> >> * the state. */ >> > >> >> >> > >> >> local_inc(&dynticks_nmi_nesting, (we incremented dynticks ? 1 : 2)); >> > >> >> >> > >> >> WARN_ON(we incremented dynticks and dynticks_nmi_nesting was nonzero); >> > >> >> } >> > >> >> >> > >> >> void rcu_nmi_exit(void) >> > >> >> { >> > >> >> WARN_ON(!(dynticks & 1)); >> > >> >> locally atomically: dynticks_nmi_nesting -= 2, unless >> > >> >> dynticks_nmi_nesting == 1, in which case set it to zero >> > >> >> >> > >> >> if (dynticks_nmi_nesting was 1) >> > >> >> atomic_inc(&dynticks); >> > >> >> } >> > >> >> >> > >> >> The invariant here is that, for a single unnested enter/exit, if >> > >> >> dynticks_nmi_nesting != 0, then dynticks is odd. As a result, an >> > >> >> rcu_nmi_enter/rcu_nmi_exit pair at any time when dynticks_nmi_nesting >> > >> >> != 0 *or* dynticks is odd will have no net effect, so the invariant, >> > >> >> in fact, holds for all invocations, nested or otherwise. >> > >> >> >> > >> >> At least one of those conditions is true at all times during the >> > >> >> execution of outermost pair, starting with the first atomic operation >> > >> >> and ending with the final atomic_inc. So they nest properly no matter >> > >> >> what else happens (unless, of course, someone else pokes dynticks in >> > >> >> the middle). >> > >> >> >> > >> >> Thoughts? >> > >> > >> > >> > Let's see... The evenness of ->dynticks should be preserved by nested NMI >> > >> > handlers, so the check and increment need not be atomic. We don't have >> > >> > any way (other than atomic operations) to do local atomic modifications >> > >> > on all architectures, because we cannot mask NMIs. (Yes, it can work >> > >> > on x86, but this is common code that needs to work everywhere.) On the >> > >> > other hand, presumably NMIs are rare, so atomic modification of the NMI >> > >> > nesting counter should be OK, at least if it proves absolutely necessary. >> > >> > And I am thinking that a mechanical proof will be needed here. :-/ >> > >> > >> > >> > But first, let me try generating the code and informally evaluating it: >> > >> > >> > >> > 1 struct rcu_dynticks { >> > >> > 2 long long dynticks_nesting; >> > >> > 3 int dynticks_nmi_nesting; >> > >> > 4 atomic_t dynticks; >> > >> > 5 }; >> > >> > 6 >> > >> > 7 void rcu_nmi_enter(void) >> > >> > 8 { >> > >> > 9 struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); >> > >> > 10 int incby = 2; >> > >> > 11 >> > >> > 12 if (!(atomic_read(&rdtp->dynticks) & 0x1)) { >> > >> > 13 smp_mb__before_atomic(); >> > >> > 14 atomic_inc(&rdtp->dynticks); >> > >> > 15 smp_mb__after_atomic(); >> > >> > 16 WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1)); >> > >> > 17 incby = 1; >> > >> >> > >> WARN_ON_ONCE(rdtp->dynticks_nmi_nesting < 1) here, perhaps? >> > > >> > > That would make sense. >> > > >> > >> > 18 } >> > >> > 19 rdtp->dynticks_nmi_nesting += incby; >> > >> >> > >> Oh, I see why you don't need local_add -- it's because an nmi in the >> > >> middle of this increment won't have any effect on the interrupted >> > >> code, so even a software RMW will be okay. >> > > >> > > Yep! ;-) >> > > >> > >> > 20 barrier(); >> > >> > 21 } >> > >> > 22 >> > >> > 23 void rcu_nmi_exit(void) >> > >> > 24 { >> > >> > 25 struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); >> > >> > 26 >> > >> > 27 WARN_ON_ONCE(!rdtp->dynticks_nmi_nesting); >> > >> > 28 WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1)); >> > >> > 29 if (rdtp->dynticks_nmi_nesting != 1) { >> > >> >> > >> WARN_ON_ONCE(rdtp->dynticks_nmi_nesting < 2), perhaps? >> > > >> > > This is already implied by the WARN_ON_ONCE() on line 27 and the check >> > > on line 29. >> > >> > I was worried about negative numbers. Maybe change line 27 to >> > WARN_ON_ONCE(rdtp->dynticks_nmi_nesting <= 0), then? (Or is it >> > unsigned? If so, let's make to signed to catch this type of error.) >> >> Good point, they are signed, so your WARN_ON_ONCE() would work. >> >> > >> > 30 rdtp->dynticks_nmi_nesting -= 2; >> > >> > 31 return; >> > >> > 32 } >> > >> > 33 rdtp->dynticks_nmi_nesting = 0; >> > >> > 34 smp_mb__before_atomic(); >> > >> >> > >> This implies barrier(), right? >> > > >> > > Yep! >> > > >> > >> > 35 atomic_inc(&rdtp->dynticks); >> > >> > 36 smp_mb__after_atomic(); >> > >> > 37 WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1); >> > >> > 38 } >> > >> > >> > >> > Line 9 picks up a pointer to this CPU's rcu_dynticks structure and line 10 >> > >> > assumes that we don't need to increment ->dynticks. >> > >> > >> > >> > Line 12 checks to see if ->dynticks is even. Note that this check is >> > >> > stable: If there are nested NMIs, they will increment ->dynticks twice >> > >> > or not at all, and either way preserves the evenness (to be proven, of >> > >> > course, but that is the plan). If ->dynticks is even, lines 13-15 >> > >> > atomically increment it, line 16 complains if still even, and line 17 >> > >> > says we will increment ->dynticks_nmi_nesting by only 1. >> > >> > >> > >> > Either way, line 19 increments ->dynticks_nmi_nesting as needed and >> > >> > line 20 keeps the compiler from getting too cute. >> > >> > >> > >> > For rcu_nmi_exit(), line 25 again picks up this CPUs rcu_dynticks >> > >> > structure. Lines 27 and 28 complain bitterly if invariants are violated. >> > >> > If line 29 finds that the value of ->dynticks_nmi_nesting is not 1, >> > >> > then line 30 subtracts 2 from ->dynticks_nmi_nesting and line 31 returns. >> > >> > >> > >> > Otherwise, line 33 sets ->dynticks_nmi_nesting to zero, lines 34-36 >> > >> > atomically increment ->dynticks with full ordering, and line 37 >> > >> > complains bitterly if ->dynticks is not even. >> > >> > >> > >> > So, if an NMI occurs before rcu_nmi_enter's atomic increment, then the >> > >> > nested NMI's rcu_nmi_enter() and rcu_nmi_exit() will think that they are >> > >> > not nested, which is the correct thing for them to think in that case. >> > >> > They will increment ->dynticks twice and restore ->dynticks_nmi_nesting >> > >> > to zero (adding and then subtracting 1). If the NMI happens after the >> > >> > atomic increment, then the nested rcu_nmi_enter() and rcu_nmi_exit() >> > >> > will leave ->dynticks alone, and will restore ->dynticks_nmi_nesting >> > >> > to zero (adding and subtracting two again). If the NMI happens after >> > >> > the increment of ->dynticks_nmi_nesting, the nested NMI's rcu_nmi_enter() >> > >> > and rcu_nmi_exit() will again restore ->dynticks_nmi_nesting, but this >> > >> > time to one (again adding and subtracting two). >> > >> > >> > >> > In rcu_nmi_exit(), ->dynticks_nmi_nesting of zero had better not happen, >> > >> > one means we need to atomically increment ->dynticks, and other values >> > >> > mean that we are partially or fully nested. Reasoning proceeds as for >> > >> > rcu_nmi_enter(), but in the opposite direction. >> > >> > >> > >> > Whew! That might even work. >> > >> >> > >> I think I like this, with the warnings above. >> > > >> > > OK with dropping the one that I called out as redundant? >> > >> > Sure, but see about. >> > >> > > >> > >> > But how about taking a different approach. Assuming that there can >> > >> > never be more than (say) 14 nesting NMI-like things, use the lower >> > >> > four bits of ->dynticks to represent the NMI nesting and the upper >> > >> > 28 bits as the counter. This of course requires modifying lots of >> > >> > places in RCU that check the counter, but it is probably time to >> > >> > abstract the check anyway. >> > >> > >> > >> > This would allow my earlier attempted logic to work and (maybe) simplify >> > >> > the reasoning a bit (and yes, the "magic" constants need macros): >> > >> > >> > >> > void rcu_nmi_enter(void) >> > >> > { >> > >> > struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); >> > >> > int nesting = atomic_read(&rdtp->dynticks) & 0xf; >> > >> > int incby = 0x01; >> > >> > >> > >> > WARN_ON_ONCE(nexting == 0xf); >> > >> > if (nesting == 0) { >> > >> > if (atomic_read(&rdtp->dynticks) & 0x10) >> > >> > return; >> > >> > incby = 0x11; >> > >> > } >> > >> > smp_mb__before_atomic(); >> > >> > atomic_add(&rdtp->dynticks, incby); >> > >> > smp_mb__after_atomic(); >> > >> > WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1)); >> > >> > } >> > >> > >> > >> > void rcu_nmi_exit(void) >> > >> > { >> > >> > struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); >> > >> > int nesting = atomic_read(&rdtp->dynticks) & 0xf; >> > >> > int incby = 0x0f; >> > >> > >> > >> > if (nesting == 0) >> > >> > return; >> > >> > if (nesting > 1) >> > >> > incby = -1; >> > >> > smp_mb__before_atomic(); >> > >> > atomic_add(&rdtp->dynticks, incby); >> > >> > smp_mb__after_atomic(); >> > >> > WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1); >> > >> > } >> > >> > >> > >> > Over to you! ;-) >> > >> >> > >> This latter one is all you :) >> > > >> > > Well, let's see how I feel about it after trying a Promela model of >> > > the first code sequence. ;-) >> > >> > :) >> > >> > Does Promela understand the differences between this type of >> > reentrancy and real threading? >> >> Not as far as I know. But it can be tricked into making this distinction. >> One thread just has the Promela code as is, and the other thread has >> the same Promela code entirely contained in an atomic block. This means >> that the entire second thread must executed at one point in the first >> thread, just like an NMI would. > > And the following Promela model claims that your approach works. > Should I trust it? ;-) >
I think so.
Want to write a patch? If so, whose tree should it go in? I can add it to my IST series, but that seems a bit odd.
--Andy
> Thanx, Paul > > ------------------------------------------------------------------------ > > /* > * Promela model for Andy Lutomirski's suggested change to rcu_nmi_enter() > * that allows nesting. > * > * 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, you can access it online at > * http://www.gnu.org/licenses/gpl-2.0.html. > * > * Copyright IBM Corporation, 2014 > * > * Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > */ > > byte dynticks_nesting; > byte dynticks_nmi_nesting; > byte dynticks; > byte busy; > > /* > * Promela verision of rcu_nmi_enter(). > */ > inline rcu_nmi_enter() > { > assert(dynticks_nmi_nesting >= 0); > if > :: (dynticks & 1) == 0 -> > atomic { > dynticks = dynticks + 1; > } > assert((dynticks & 1) == 1); > dynticks_nmi_nesting = dynticks_nmi_nesting + 1; > assert(dynticks_nmi_nesting >= 1); > :: else -> > dynticks_nmi_nesting = dynticks_nmi_nesting + 2; > fi; > } > > /* > * Promela verision of rcu_nmi_exit(). > */ > inline rcu_nmi_exit() > { > assert(dynticks_nmi_nesting > 0); > assert((dynticks & 1) != 0); > if > :: dynticks_nmi_nesting != 1 -> > dynticks_nmi_nesting = dynticks_nmi_nesting - 2; > :: else -> > dynticks_nmi_nesting = 0; > atomic { > dynticks = dynticks + 1; > } > assert((dynticks & 1) == 0); > fi; > } > > /* > * Base-level NMI runs non-atomically. Crudely emulates process-level > * dynticks-idle entry/exit. > */ > proctype base_NMI() > { > do > :: if > :: 1 -> atomic { > dynticks = dynticks + 1; > } > busy = 0; > :: 1 -> skip; > fi; > rcu_nmi_enter(); > assert((dynticks & 1) == 1); > rcu_nmi_exit(); > if > :: busy -> skip; > :: !busy -> > atomic { > dynticks = dynticks + 1; > } > busy = 1; > fi; > od; > } > > /* > * Nested NMI runs atomically to emulate interrupting base_level(). > */ > proctype nested_NMI() > { > do > :: atomic { > rcu_nmi_enter(); > assert((dynticks & 1) == 1); > rcu_nmi_exit(); > } > od; > } > > init { > dynticks_nesting = 0; > dynticks_nmi_nesting = 0; > dynticks = 0; > busy = 0; > run base_NMI(); > run nested_NMI(); > } >
-- Andy Lutomirski AMA Capital Management, LLC
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