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

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Date:   Sat, 20 Mar 2021 00:47:28 +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 04:38:48PM -0700, Paul E. McKenney wrote:
> > 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.
> 
> You mean like this?
> 
> 	oldstate = start_poll_synchronize_rcu();
> 	// Why wait?  Beat the rush!!!
> 	cond_synchronize_rcu(oldstate);
> 
> This would be a bit silly (why not just call synchronize_rcu()?),
> and yes, this would unconditionally get you an extra RCU grace period.
> Then again, any call to cond_synchronize_rcu() before the desired grace
> period has expired will get you an extra grace period, and maybe more.
> 
> So a given use case either needs to not care about the added latency
> or have a high probability of invoking cond_synchronize_rcu() after
> the desired grace period has expired.

Fair point!

Thanks.