Date:	Mon, 24 Nov 2014 13:27:55 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Tony Luck <tony.luck@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	Josh Triplett <josh@...htriplett.org>,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH v4 2/5] x86, traps: Track entry into and exit from IST
 context

On Mon, Nov 24, 2014 at 12:54:41PM -0800, Paul E. McKenney 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@...ux.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@...ux.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@...ux.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.

I take it back.  We get here only if there is no lower-level NMI in effect.
I could do WARN_ON_ONCE(rdtp->dynticks_nmi_nesting > 0), though.

							Thanx, Paul

> > >         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.
> 
> > >         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?
> 
> > > 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.  ;-)
> 
> 							Thanx, Paul
> 
> > --Andy
> > 
> > >
> > >                                                         Thanx, Paul
> > >
> > 
> > 
> > 
> > -- 
> > Andy Lutomirski
> > AMA Capital Management, LLC
> > 

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