linux-kernel - Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier

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Message-Id: <20171011173423.GO3521@linux.vnet.ibm.com>
Date:   Wed, 11 Oct 2017 10:34:23 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     David Howells <dhowells@...hat.com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        torvalds@...ux-foundation.org, mark.rutland@....com,
        linux-arch@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Alexander Kuleshov <kuleshovmail@...il.com>, dvyukov@...gle.com
Subject: Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove
 smp_read_barrier_depends()

On Wed, Oct 11, 2017 at 07:11:37PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 11, 2017 at 10:06:31AM -0700, Paul E. McKenney wrote:
> 
> > This, you mean?
> > 
> > data = rcu_dereference_sched(*this_cpu_ptr(&cpufreq_update_util_data));
> 
> Yep, that one. Although in my tree it now appears to look like:
> 
>         data = rcu_dereference_sched(*per_cpu_ptr(&cpufreq_update_util_data,
>                                                   cpu_of(rq)));

So in the non-RCU case, we could simply replace rcu_dereference_sched()
with ACCESS_ONCE(), right?  Thus no need to change the per-CPU
primitives.  Especially given that most uses of per-CPU variables
don't even need protection from load/store tearing, let alone
protection from Alpha.

Or am I missing something subtle here?

							Thanx, Paul