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 16:17:25 +0100
From:   David Howells <dhowells@...hat.com>
To:     Will Deacon <will.deacon@....com>
Cc:     dhowells@...hat.com, paulmck@...ux.vnet.ibm.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>
Subject: Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends()

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.

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.

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()?

Also, does WRITE_ONCE() imply a preceding barrier?

David