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 18:24:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 06:12:20PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 11, 2017 at 08:59:48AM -0700, Paul E. McKenney wrote:
> > On Wed, Oct 11, 2017 at 04:17:25PM +0100, David Howells wrote:
> > > I will, however, quibble with the appropriateness of the name READ_ONCE()...
> > > I still think it's not sufficiently obvious that this is a barrier and the
> > > barrier is after.  Maybe READ_AND_BARRIER()?
> > 
> > Linus was unhappy with READ_ONCE_CTRL() to tag control dependencies, but
> > indicated that he might consider it if it helped code-analysis tools.
> > Adding Dmitry Vyukov for his thoughts on whether tagging READ_ONCE()
> > for dependencies would help.  Me, I would suggest READ_ONCE_DEP(), but
> > let's figure out if the bikeshed needs to be painted before arguing over
> > the color.  ;-)
> 
> Count me one vote for the READ_ONCE() name. This is about dependent
> reads, which are nothing special on anything except Alpha.
> 
> We want to remove the exception/specialness from the memory model; and
> therefore have to fix up all primitives that could possibly be used for
> these reads to unconditionally issue the barrier (on Alpha). The
> alternative is: rm -rf arch/alpha.
> 
> Adding something like READ_ONCE_DEP() does not rid us of the idea that
> dependent reads are special and thus defeats the purpose, we might as
> well retain lockless_dereference().
> 
> Now; any user of dependent reads must use READ_ONCE() in any case, to
> avoid load tearing and reloads. So using READ_ONCE() for the dependent
> reads is not extra or additional (note we'll also have to add the
> barrier to all our relaxed and release atomics and anything else that
> implies READ_ONCE and doesn't already imply smp_mb() after).

Add the per-cpu ops to that list, they imply READ_ONCE(). Consider for
example this example:


	for_each_possible_cpu(cpu)
		smp_store_release(per_cpu_ptr(&foo, cpu), obj);

-vs-

	obj = this_cpu_read(foo);
	if (obj->ponies)
		fart_rainbow(obj);