linux-kernel - Re: [PATCH RFC v2 rcu] Fix get_state_synchronize_rcu

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Message-ID: <a18ec931-a7cb-408b-8707-f88068a13ba9@paulmck-laptop>
Date: Sat, 25 Jan 2025 10:39:01 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org
Subject: Re: [PATCH RFC v2 rcu] Fix get_state_synchronize_rcu_full() GP-start
 detection

On Sat, Jan 25, 2025 at 03:56:16PM +0100, Frederic Weisbecker wrote:
> Le Fri, Jan 24, 2025 at 04:01:55PM -0800, Paul E. McKenney a écrit :
> > On Sat, Jan 25, 2025 at 12:03:58AM +0100, Frederic Weisbecker wrote:
> > > Le Fri, Dec 13, 2024 at 11:49:49AM -0800, Paul E. McKenney a écrit :
> > > > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> > > > index 2f9c9272cd486..d2a91f705a4ab 100644
> > > > --- a/kernel/rcu/rcu.h
> > > > +++ b/kernel/rcu/rcu.h
> > > > @@ -162,7 +162,7 @@ static inline bool rcu_seq_done_exact(unsigned long *sp, unsigned long s)
> > > >  {
> > > >  	unsigned long cur_s = READ_ONCE(*sp);
> > > >  
> > > > -	return ULONG_CMP_GE(cur_s, s) || ULONG_CMP_LT(cur_s, s - (2 * RCU_SEQ_STATE_MASK + 1));
> > > > +	return ULONG_CMP_GE(cur_s, s) || ULONG_CMP_LT(cur_s, s - (3 * RCU_SEQ_STATE_MASK + 1));
> > > 
> > > This might need a comment.
> > 
> > Good point!  Would you like to propose one?
> 
> Ok.

Sounds good!

> > > The way I understand it is that rcu_state.gp_seq might be seen started while
> > > root_rnp->gp_seq is not. So rcu_seq_snap() on the started rcu_state.gp_seq
> > > may return maximum 2 full GPs ahead of root_rnp->gp_seq. And therefore it takes below
> > > 2 GPs to safely deduce we wrapped around.
> > 
> > Exactly!
> > 
> > > Should it be ULONG_CMP_LT(cur_s, s - (2 * (RCU_SEQ_STATE_MASK + 1))) ?
> > 
> > Quite possibly.  I freely admit that I allowed a bit of slop because
> > time was of the essence (holidays and all that) and also it does not
> > hurt much to lose a couple of counts out of a 2^32 cycle, to say nothing
> > of the common-case 2^64 cycle.  It would not hurt to be exact, but it
> > would be necessary to convince ourselves that we were not off by one in
> > the wrong direction.
> > 
> > I would be happy to see a patch, as long as it was sufficiently
> > convincing.
> 
> I'm not so much concerned about being exact but rather about making
> sure we still understand what we did within one year. We can leave one
> more grace period than what we expect out of paranoia but, the most
> important is that we comment about what we expect and why. Let me
> prepare a patch for that.

Even better!

> In the meantime for your patch:
> 
>    Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Thank you, and I will apply this on my next rebase.

							Thanx, Paul

> > > Or am I missing something?
> > 
> > Not that I can see.  So the answer is probably "yes".  ;-)
> 
> Thanks! :-)
> 
> > 
> > 							Thanx, Paul