Re: rcu: WARNING in rcu_seq_end

[Boqun Feng]

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.

> > 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.

> > 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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