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Date: Wed, 11 Oct 2017 18:17:52 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: David Howells <dhowells@redhat.com>
Cc: paulmck@linux.vnet.ibm.com, Will Deacon <will.deacon@arm.com>,
        linux-kernel@vger.kernel.org, mingo@kernel.org,
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        linux-arch@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
        Alexander Kuleshov <kuleshovmail@gmail.com>, dvyukov@google.com
Subject: Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove
 smp_read_barrier_depends()
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On Wed, Oct 11, 2017 at 05:07:05PM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> 
> > It does not.  In most cases, the barriered version would be
> > smp_store_release().
> 
> Ummm... Is that good enough?  Is:
> 
> 	WRITE_ONCE(x, 1);
> 	WRITE_ONCE(x, 2);
> 
> equivalent to:
> 
> 	smp_store_release(x, 1);
> 	smp_store_release(x, 2);
> 
> if CONFIG_SMP=n?

Almost; it ends up being:

	barrier();
	WRITE_ONCE(x, 1);
	barrier();
	WRITE_ONCE(x, 2);

> (Consider what happens if an interrupt messes with x).
> 
> If it is good enough, should we be using smp_load_acquire() rather than
> READ_ONCE()?

No, smp_load_acquire() is strictly stronger (and far more expensive on
!Alpha).

Dependent loads do not require barriers (except Alpha, and we want to
kill that special case).