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 08:59:48 -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 04:17:25PM +0100, David Howells wrote:
> Will Deacon <will.deacon@....com> wrote:
> 
> > FWIW, that's exactly what my patches do, this fixup looks a bit weird
> > because it removes a prior barrier which suggests that either (a) it's in
> > the wrong place to start with, or (b) we're annotating the wrong load.
> 
> There is a loop involved.  The barrier is against the read in the previous
> iteration of the loop.  IIRC, the reason I did it this way is to avoid the
> need for the barrier if there's nothing on the 'after-side' - ie. we examine
> the pointer and see that it's NULL or a leaf.  However, I'm not sure that's a
> particularly necessary optimisation.

Given that smp_read_barrier_depends() is nothingness on anything other
than DEC Alpha, I would argue that this optimization is not necessary.

> So if READ_ONCE() issues a smp_read_barrier_depends() after the read, then
> I've no problem with the removal of these explicit barriers.

Very good!

> 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.  ;-)

> Also, does WRITE_ONCE() imply a preceding barrier?

It does not.  In most cases, the barriered version would be
smp_store_release().

							Thanx, Paul