linux-kernel - Re: [PATCH tip/core/rcu 16/27] rcu: Add comment documenting how rcu_seq

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Message-Id: <20180626193146.GK3593@linux.vnet.ibm.com>
Date:   Tue, 26 Jun 2018 12:31:46 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 16/27] rcu: Add comment documenting how
 rcu_seq_snap works

On Tue, Jun 26, 2018 at 09:21:13PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 26, 2018 at 11:08:55AM -0700, Paul E. McKenney wrote:
> > On Tue, Jun 26, 2018 at 07:14:54PM +0200, Peter Zijlstra wrote:
> > > On Mon, Jun 25, 2018 at 05:35:02PM -0700, Paul E. McKenney wrote:
> > > > From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> > > > 
> > > > rcu_seq_snap may be tricky to decipher. Lets document how it works with
> > > > an example to make it easier.
> > > 
> > > Since you had me looking at them functions; why isn't rcu_seq_snap()
> > > using smp_load_acquire() and rcu_seq_end() using smp_store_release() ?
> > > Their respective comments seem to suggest that would be sufficent.
> > 
> > I do not believe that this would suffice.  Would it make sense to refer
> > to Documentation/RCU/Design/Memory-Ordering in the comment header?
> 
> No, because I can't read that thing in an editor.
> 
> > Except that this would invite sprinkling this pathname far and wide...
> > 
> > The key point is that these functions are part of the any-to-any
> > memory-ordering guarantee that RCU grace periods provide.
> 
> Then the existing comment is misleading and really needs change.

Would it be sufficient to add something like "The memory barrier is
required to support the many-to-many ordering guaranteed by RCU grace
periods"?

							Thanx, Paul