linux-kernel - Re: [PATCH] refcount_t: documentation for memory ordering differences

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Message-ID: <2dbef3c8-a70f-9584-2de5-3440520f224a@infradead.org>
Date:   Fri, 1 Dec 2017 12:34:23 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Elena Reshetova <elena.reshetova@...el.com>, peterz@...radead.org
Cc:     linux-kernel@...r.kernel.org, keescook@...omium.org,
        david@...morbit.com
Subject: Re: [PATCH] refcount_t: documentation for memory ordering differences

On 11/29/2017 04:36 AM, Elena Reshetova wrote:
> Some functions from refcount_t API provide different
> memory ordering guarantees that their atomic counterparts.
> This adds a document outlining these differences.
> 
> Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
> ---
>  Documentation/core-api/index.rst              |   1 +
>  Documentation/core-api/refcount-vs-atomic.rst | 129 ++++++++++++++++++++++++++
>  2 files changed, 130 insertions(+)
>  create mode 100644 Documentation/core-api/refcount-vs-atomic.rst

> diff --git a/Documentation/core-api/refcount-vs-atomic.rst b/Documentation/core-api/refcount-vs-atomic.rst
> new file mode 100644
> index 0000000..5619d48
> --- /dev/null
> +++ b/Documentation/core-api/refcount-vs-atomic.rst
> @@ -0,0 +1,129 @@
> +===================================
> +refcount_t API compared to atomic_t
> +===================================
> +
> +The goal of refcount_t API is to provide a minimal API for implementing
> +an object's reference counters. While a generic architecture-independent
> +implementation from lib/refcount.c uses atomic operations underneath,
> +there are a number of differences between some of the refcount_*() and
> +atomic_*() functions with regards to the memory ordering guarantees.
> +This document outlines the differences and provides respective examples
> +in order to help maintainers validate their code against the change in
> +these memory ordering guarantees.
> +
> +memory-barriers.txt and atomic_t.txt provide more background to the
> +memory ordering in general and for atomic operations specifically.
> +
> +Relevant types of memory ordering
> +=================================
> +
> +**Note**: the following section only covers some of the memory
> +ordering types that are relevant for the atomics and reference
> +counters and used through this document. For a much broader picture
> +please consult memory-barriers.txt document.
> +
> +In the absence of any memory ordering guarantees (i.e. fully unordered)
> +atomics & refcounters only provide atomicity and
> +program order (po) relation (on the same CPU). It guarantees that
> +each atomic_*() and refcount_*() operation is atomic and instructions
> +are executed in program order on a single CPU.
> +This is implemented using READ_ONCE()/WRITE_ONCE() and
> +compare-and-swap primitives.
> +
> +A strong (full) memory ordering guarantees that all prior loads and
> +stores (all po-earlier instructions) on the same CPU are completed
> +before any po-later instruction is executed on the same CPU.
> +It also guarantees that all po-earlier stores on the same CPU
> +and all propagated stores from other CPUs must propagate to all
> +other CPUs before any po-later instruction is executed on the original
> +CPU (A-cumulative property). This is implemented using smp_mb().

I don't know what "A-cumulative property" means, and google search didn't
either.

Is it non-cumulative, similar to typical vs. atypical, where atypical
roughly means non-typical.  Or is it accumlative (something being
accumulated, summed up, gathered up)?

Or is it something else.. TBD?

> +A RELEASE memory ordering guarantees that all prior loads and
> +stores (all po-earlier instructions) on the same CPU are completed
> +before the operation. It also guarantees that all po-earlier
> +stores on the same CPU and all propagated stores from other CPUs
> +must propagate to all other CPUs before the release operation
> +(A-cumulative property). This is implemented using smp_store_release().

thanks.
-- 
~Randy