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

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Date:   Wed, 11 Oct 2017 09:19:33 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     David Howells <dhowells@...hat.com>
Cc:     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,
        peterz@...radead.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 05:07:05PM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@...ux.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?

	smp_store_release(&x, 1);
	smp_store_release(&x, 2);

But yes, give or take that smp_store_release() potentially disables
more compiler optimizations than does WRITE_ONCE().

> (Consider what happens if an interrupt messes with x).

OK, I will bite...  What is your scenario in which an interrupt
gives different results for CONFIG_SMP=n?  The barriers

> If it is good enough, should we be using smp_load_acquire() rather than
> READ_ONCE()?

On x86, that might be OK, give or take that smp_load_acquire() potentially
disables more optimizations than does READ_ONCE().  But on ARM, PowerPC,
MIPS, and so on, smp_load_acquire() emits a memory-barrier instruction
and READ_ONCE() does not.

							Thanx, Paul