linux-kernel - Re: [PATCH tip/core/rcu 1/3] rcu: Provide polling interfaces for Tree RCU grace periods

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Message-ID: <20210319221040.GC814853@lothringen>
Date:   Fri, 19 Mar 2021 23:10:40 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 1/3] rcu: Provide polling interfaces for
 Tree RCU grace periods

On Fri, Mar 19, 2021 at 10:51:16AM -0700, Paul E. McKenney wrote:
> On Fri, Mar 19, 2021 at 02:58:54PM +0100, Frederic Weisbecker wrote:
> > It's all a matter of personal taste but if I may suggest some namespace
> > modifications:
> > 
> > get_state_synchronize_rcu() -> synchronize_rcu_poll_start_raw()
> > start_poll_synchronize_rcu() -> synchronize_rcu_poll_start()
> > poll_state_synchronize_rcu() -> synchronize_rcu_poll()
> > cond_synchronize_rcu() -> synchronize_rcu_cond()
> > 
> > But it's up to you really.
> 
> I am concerned about starting anything "synchronize_rcu" if that
> thing doesn't unconditionally wait for a grace period.  "What do
> you mean that there was no grace period?  Don't you see that call to
> synchronize_rcu_poll_start_raw()???"

I see, that could indeed be confusing.

> 
> This objection doesn't apply to cond_synchronize_rcu(), but it is
> already in use, so any name change should be worked with the users.
> All two of them.  ;-)

Probably not worth it. We have cond_resched() as a schedule() counterpart
for a reference after all.

> > >  /**
> > > + * start_poll_state_synchronize_rcu - Snapshot and start RCU grace period
> > > + *
> > > + * Returns a cookie that is used by a later call to cond_synchronize_rcu()
> > 
> > It may be worth noting that calling start_poll_synchronize_rcu() and then
> > pass the cookie to cond_synchronize_rcu() soon after may end up waiting for
> > one more grace period.
> 
> You mean this sequence of events?
> 
> 1.	cookie = start_poll_synchronize_rcu()
> 
> 2.	The grace period corresponding to cookie is almost over...
> 
> 3.	cond_synchronize_rcu() checks the cookie and sees that the
> 	grace period has not yet expired.
> 
> 4.	The grace period corresponding to cookie completes.
> 
> 5.	Someone else starts a grace period.
> 
> 6.	cond_synchronize_rcu() invokes synchronize_rcu(), which waits
> 	for the just-started grace period plus another grace period.
> 	Thus, there has been no fewer than three full grace periods
> 	between the call to start_poll_synchronize_rcu() and the
> 	return from cond_synchronize_rcu().
> 
> Yes, this can happen!  And it can be worse, for example, it is quite
> possible that cond_synchronize_rcu() would be preempted for multiple
> grace periods at step 5, in which case it would still wait for almost
> two additional grace periods.
> 
> Or are you thinking of something else?

I didn't even think that far.
My scenario was:

1.	cookie = start_poll_synchronize_rcu()
 
 
2.	cond_synchronize_rcu() checks the cookie and sees that the
	grace period has not yet expired. So it calls synchronize_rcu()
	which queues a callback.

3.	The grace period for the cookie eventually completes.

4.	The callback queued in 2. gets assigned a new grace period number.
	That new grace period starts.

5.	The new grace period completes and synchronize_rcu() returns.


But I think this is due to some deep misunderstanding from my end.


> > > + * If a full RCU grace period has elapsed since the earlier call from
> > > + * which oldstate was obtained, return @true, otherwise return @false.
> > > + * Otherwise, invoke synchronize_rcu() to wait for a full grace period.
> > 
> > Rephrase suggestion for the last sentence:
> > 
> > "In case of failure, it's up to the caller to try polling again later or
> > invoke synchronize_rcu() to wait for a new full grace period to complete."
> 
> How about like this?
> 
> /**
>  * poll_state_synchronize_rcu - Conditionally wait for an RCU grace period
>  *
>  * @oldstate: return from call to get_state_synchronize_rcu() or start_poll_synchronize_rcu()
>  *
>  * If a full RCU grace period has elapsed since the earlier call from
>  * which oldstate was obtained, return @true, otherwise return @false.
>  * If @false is returned, it is the caller's responsibilty to invoke this
>  * function later on until it does return @true.  Alternatively, the caller
>  * can explicitly wait for a grace period, for example, by passing @oldstate
>  * to cond_synchronize_rcu() or by directly invoking synchronize_rcu().

Yes very nice!

Thanks!