Re: rcu: WARNING in rcu_seq_end

[Boqun Feng]

On Tue, Mar 07, 2017 at 03:31:54PM -0800, Paul E. McKenney wrote:
> On Wed, Mar 08, 2017 at 07:05:13AM +0800, Boqun Feng wrote:
> > On Tue, Mar 07, 2017 at 07:27:15AM -0800, Paul E. McKenney wrote:
> > > On Tue, Mar 07, 2017 at 03:43:42PM +0100, Dmitry Vyukov wrote:
> > > > On Tue, Mar 7, 2017 at 3:27 PM, Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
> > > > > On Tue, Mar 07, 2017 at 08:05:19AM +0100, Dmitry Vyukov wrote:
> > > > > [...]
> > > > >> >>
> > > > >> >> What is that mutex? And what locks/unlocks provide synchronization? I
> > > > >> >> see that one uses exp_mutex and another -- exp_wake_mutex.
> > > > >> >
> > > > >> > Both of them.
> > > > >> >
> > > > >> > ->exp_mutex is acquired by the task requesting the grace period, and
> > > > >> > the counter's first increment is done by that task under that mutex.
> > > > >> > This task then schedules a workqueue, which drives forward the grace
> > > > >> > period.  Upon grace-period completion, the workqueue handler does the
> > > > >> > second increment (the one that your patch addressed).  The workqueue
> > > > >> > handler then acquires ->exp_wake_mutex and wakes the task that holds
> > > > >> > ->exp_mutex (along with all other tasks waiting for this grace period),
> > > > >> > and that task releases ->exp_mutex, which allows the next grace period to
> > > > >> > start (and the first increment for that next grace period to be carried
> > > > >> > out under that lock).  The workqueue handler releases ->exp_wake_mutex
> > > > >> > after finishing its wakeups.
> > > > >>
> > > > >> Then we need the following for the case when task requesting the grace
> > > > >> period does not block, right?
> > > > >
> > > > > Won't be necessary I think, as the smp_mb() in rcu_seq_end() and the
> > > > > smp_mb__before_atomic() in sync_exp_work_done() already provide the
> > > > > required ordering, no?
> > > > 
> > > > smp_mb() is probably fine, but smp_mb__before_atomic() is release not
> > > > acquire. If we want to play that game, then I guess we also need
> > 
> > The point is that smp_mb__before_atomic() + atomic_long_inc() will
> > guarantee a smp_mb() before or right along with the atomic operation,
> > and that's enough because rcu_seq_done() followed by a smp_mb() will
> > give it a acquire-like behavior.
> 
> Given current architectures, true enough, from what I can see.
> 
> However, let's take a look at atomic_ops.rst:
> 
> 
> 	If a caller requires memory barrier semantics around an atomic_t
> 	operation which does not return a value, a set of interfaces are
> 	defined which accomplish this::
> 
> 		void smp_mb__before_atomic(void);
> 		void smp_mb__after_atomic(void);
> 
> 	For example, smp_mb__before_atomic() can be used like so::
> 
> 		obj->dead = 1;
> 		smp_mb__before_atomic();
> 		atomic_dec(&obj->ref_count);
> 
> 	It makes sure that all memory operations preceding the atomic_dec()
> 	call are strongly ordered with respect to the atomic counter
> 	operation.  In the above example, it guarantees that the assignment of
> 	"1" to obj->dead will be globally visible to other cpus before the
> 	atomic counter decrement.
> 
> 	Without the explicit smp_mb__before_atomic() call, the
> 	implementation could legally allow the atomic counter update visible
> 	to other cpus before the "obj->dead = 1;" assignment.
> 
> So the ordering is guaranteed against the atomic operation, not
> necessarily the stuff after it.  But again, the implementations I know
> of do make the guarantee, hence my calling it a theoretical bug in the
> commit log.
> 

Fair enough ;-) It's me who misunderstood this part of document.

However, the names of the barriers are smp_mb__{before,after}_atomic(),
so if they, semantically, only provide ordering for the corresponding
atomic ops rather than a full barrier, I would their names are
misleading ;-)

> > > > smp_mb__after_atomic() there. But it would be way easier to understand
> > 
> > Adding smp_mb__after_atomic() would be pointless as it's the load of 
> > ->expedited_sequence that we want to ensure having acquire behavior
> > rather than the atomic increment of @stat.
> 
> Again, agreed given current code, but atomic_ops.rst doesn't guarantee
> ordering past the actual atomic operation itself.
> 

Neither does atomic_ops.rst guarantee the ordering between a load before
the atomic op and memory accesses after the atomic op, right? I.e.
atomic_ops.rst doesn't say no for reordering like this:

	r1 = READ_ONCE(a); ---------+
	atomic_long_inc(b);         |              
	smp_mb__after_atomic();     |
	WRITE_ONCE(c);              |
	{r1 = READ_ONCE(a)} <-------+
	
So it's still not an acquire for READ_ONCE(a), in our case "a" is
->expedited_sequence.

To me, we can either fix the atomic_ops.rst or, as I proposed, just
change smp_mb__before_atomic() to smp_mb().

Thoughts?

Regards,
Boqun

> 							Thanx, Paul
> 
> > > > what's happens there and prove that it's correct, if we use
> > > > store_release/load_acquire.
> > > 
> > > Fair point, how about the following?
> > > 
> > > 							Thanx, Paul
> > > 
> > > ------------------------------------------------------------------------
> > > 
> > > commit 6fd8074f1976596898e39f5b7ea1755652533906
> > > Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > Date:   Tue Mar 7 07:21:23 2017 -0800
> > > 
> > >     rcu: Add smp_mb__after_atomic() to sync_exp_work_done()
> > >     
> > >     The sync_exp_work_done() function needs to fully order the counter-check
> > >     operation against anything happening after the corresponding grace period.
> > >     This is a theoretical bug, as all current architectures either provide
> > >     full ordering for atomic operation on the one hand or implement,
> > >     however, a little future-proofing is a good thing.  This commit
> > >     therefore adds smp_mb__after_atomic() after the atomic_long_inc()
> > >     in sync_exp_work_done().
> > >     
> > >     Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > >     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > 
> > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > > index 027e123d93c7..652071abd9b4 100644
> > > --- a/kernel/rcu/tree_exp.h
> > > +++ b/kernel/rcu/tree_exp.h
> > > @@ -247,6 +247,7 @@ static bool sync_exp_work_done(struct rcu_state *rsp, atomic_long_t *stat,
> > >  		/* Ensure test happens before caller kfree(). */
> > >  		smp_mb__before_atomic(); /* ^^^ */
> > >  		atomic_long_inc(stat);
> > > +		smp_mb__after_atomic(); /* ^^^ */
> > 
> > If we really care about future-proofing, I think it's more safe to
> > change smp_mb__before_atomic() to smp_mb() rather than adding
> > __after_atomic() barrier. Though I think both would be unnecessary ;-)
> > 
> > Regards,
> > Boqun
> > 
> > >  		return true;
> > >  	}
> > >  	return false;
> > > 
> 
>

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