Date:   Wed, 29 Nov 2017 17:36:00 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Elena Reshetova <elena.reshetova@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dave Chinner <david@...morbit.com>, linux-doc@...r.kernel.org,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH] refcount_t: documentation for memory ordering differences

On Wed, Nov 29, 2017 at 4:36 AM, Elena Reshetova
<elena.reshetova@...el.com> 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>

Thanks for the improvements!

I have some markup changes to add, but I'll send that as a separate patch.

Acked-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  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/index.rst b/Documentation/core-api/index.rst
> index d5bbe03..d4d54b0 100644
> --- a/Documentation/core-api/index.rst
> +++ b/Documentation/core-api/index.rst
> @@ -14,6 +14,7 @@ Core utilities
>     kernel-api
>     assoc_array
>     atomic_ops
> +   refcount-vs-atomic
>     cpu_hotplug
>     local_ops
>     workqueue
> 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().
> +
> +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().
> +
> +A control dependency (on success) for refcounters guarantees that
> +if a reference for an object was successfully obtained (reference
> +counter increment or addition happened, function returned true),
> +then further stores are ordered against this operation.
> +Control dependency on stores are not implemented using any explicit
> +barriers, but rely on CPU not to speculate on stores. This is only
> +a single CPU relation and provides no guarantees for other CPUs.
> +
> +
> +Comparison of functions
> +=======================
> +
> +case 1) - non-"Read/Modify/Write" (RMW) ops
> +-------------------------------------------
> +
> +Function changes:
> +                atomic_set() --> refcount_set()
> +                atomic_read() --> refcount_read()
> +
> +Memory ordering guarantee changes:
> +                none (both fully unordered)
> +
> +case 2) - increment-based ops that return no value
> +--------------------------------------------------
> +
> +Function changes:
> +                atomic_inc() --> refcount_inc()
> +                atomic_add() --> refcount_add()
> +
> +Memory ordering guarantee changes:
> +                none (both fully unordered)
> +
> +
> +case 3) - decrement-based RMW ops that return no value
> +------------------------------------------------------
> +Function changes:
> +                atomic_dec() --> refcount_dec()
> +
> +Memory ordering guarantee changes:
> +                fully unordered --> RELEASE ordering
> +
> +
> +case 4) - increment-based RMW ops that return a value
> +-----------------------------------------------------
> +
> +Function changes:
> +                atomic_inc_not_zero() --> refcount_inc_not_zero()
> +                no atomic counterpart --> refcount_add_not_zero()
> +
> +Memory ordering guarantees changes:
> +                fully ordered --> control dependency on success for stores
> +
> +*Note*: we really assume here that necessary ordering is provided as a result
> +of obtaining pointer to the object!
> +
> +
> +case 5) - decrement-based RMW ops that return a value
> +-----------------------------------------------------
> +
> +Function changes:
> +                atomic_dec_and_test() --> refcount_dec_and_test()
> +                atomic_sub_and_test() --> refcount_sub_and_test()
> +                no atomic counterpart --> refcount_dec_if_one()
> +                atomic_add_unless(&var, -1, 1) --> refcount_dec_not_one(&var)
> +
> +Memory ordering guarantees changes:
> +                fully ordered --> RELEASE ordering + control dependency
> +
> +Note: atomic_add_unless() only provides full order on success.
> +
> +
> +case 6) - lock-based RMW
> +------------------------
> +
> +Function changes:
> +
> +                atomic_dec_and_lock() --> refcount_dec_and_lock()
> +                atomic_dec_and_mutex_lock() --> refcount_dec_and_mutex_lock()
> +
> +Memory ordering guarantees changes:
> +                fully ordered --> RELEASE ordering + control dependency +
> +                                  hold spin_lock() on success
> --
> 2.7.4
>



-- 
Kees Cook
Pixel Security

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